Some mistake by ambassador reversed, perhaps (5)
I believe the answer is:
maybe
'perhaps' is the definition.
(synonyms)
'some mistake by ambassador reversed' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'reversed' shows that the letters should be reversed in order.
'ebyam' can be found hidden inside 'mistake by ambassador'.
'ebyam' backwards is 'MAYBE'.
(Other definitions for maybe that I've seen before include "Perhaps, might happen" , "Perhaps, possibly" .)