One's got on in an endeavour to go with the flow (9)
I believe the answer is:
bandwagon
'flow' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'one's got on in an endeavour to go with' is the wordplay.
I cannot really understand how this works, but
'on' is present in the answer.
'an' could be 'a' and 'a' is found within the answer.
'with' could be 'w' (abbreviation) and 'w' is present in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is present in the answer.
This explanation may well be incorrect...
'the' acts as a link.
(Other definitions for bandwagon that I've seen before include "vehicle for musicians" , "Fashionable movement" , "everybody wants to be on it" , "latest trend" , "Suddenly fashionable cause or activity" .)