You wrote:
Since the apa6 class automatically sets the style for citations ...
This claim isn't quite correct. If you load the apa6 document class without one of the options biblatex, apacite, or natbib, no citation management package is loaded and no bibliography style is chosen.
Here's an excerpt from page 10 of the user guide of the apa6 document class (highlighting added):

Maybe slightly counterintuitively, then, in order to achieve your formatting objectives you should not specify the natbib option while executing \documentclass, since doing so loads the apacite citation management package and the apacite bibliography style -- which is precisely what you don't want to do.
What to do? (a) Drop the natbib option from the \documentclass directive, (b) load natbib along with suitable options via a \usepackage statement, and (c) issue a suitable \bibliographystyle directive. For instance,
\usepackage[numbers]{natbib} % numeric-style citation call-outs
\bibliographystyle{acmtrans} % or some other suitable bib style
A full MWE (minimum working example) and its output:

\documentclass{apa6}
% Create a bib file "on the fly"
\begin{filecontents}[overwrite]{mybib.bib}
@misc{aa:3001,author="Anne Author",title="Thoughts",year=3001}
\end{filecontents}
\usepackage[numbers]{natbib}
\bibliographystyle{acmtrans} % or some other suitable bib style
\begin{document}
\cite{aa:3001}
\bibliography{mybib}
\end{document}
natbibpackage, and which bibliography style do you employ? – Mico Jun 18 '20 at 05:20\documentclass[a4paper,man]{apa6}\usepackage[numbers]{natbib}\bibliographystyle{acmtrans}And yes, I did a full compile cycle twice over (FWIW, I'm doing this on Overleaf not on my desktop client)
– wrahool Jun 18 '20 at 05:24\shorttitlenot being defined. Don't ignore such messages. Specifying themanoption tells LaTeX to execute, among other things,\RequirePackage[natbibapa,bibnewpage]{apacite}. This clashes with the subsequent\usepackage[numbers]{natbib}directive. That's not what you want, is it? The solution: don't specify themanoption. – Mico Jun 18 '20 at 05:38