Son engaged in reverse of best job (4)
I believe the answer is:
post
'job' is the definition.
(post is a kind of job)
'son engaged in reverse of best' is the wordplay.
'son' becomes 's'.
'engaged in' is an insertion indicator.
'reverse of' is a reversal indicator.
'best' becomes 'top' ('top' can be similar in meaning to 'best').
'top' backwards is 'pot'.
's' placed inside 'pot' is 'POST'.
(Other definitions for post that I've seen before include "a placement: it's > part of one's goal" , "Put a letter in the box - stop it" , "Stake, job or mail" , "(Holiday) communication" , "Member < to comment on website" .)