Be best man? (5)
I believe the answer is:
excel
'be best' is the definition.
(thesaurus)
'man?' is the wordplay.
I cannot really understand how this works, but
'man?' could be 'ex' (ex is a kind of man) and 'ex' is found within the answer.
The remaining letters 'cel' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for excel that I've seen before include "Be superior to" , "Eclipse" , "surpass all others" , "Every one" , "Be exceptionally good (at a subject)" .)