My Sean becomes sycophant (3,3)
I believe the answer is:
yes man
'sycophant' is the definition.
(I've seen this before)
'my sean becomes' is the wordplay.
'becomes' indicates anagramming the letters.
'my'+'sean'='mysean'
'mysean' is an anagram of 'YES-MAN'.
(Other definitions for yes man that I've seen before include "Fawning staffer" , "Toady" , "One always agreeing with a superior" , "An agreeable sycophant?" , "Obsequious male who always agrees with the boss" .)