Though beneficial, a burden to a bighead (5)
I believe the answer is:
bonus
'though beneficial a' is the definition.
I can't tell whether this defines the answer.
'burden to a bighead' is the wordplay.
'burden' becomes 'onus' (synonyms).
'to' says to put letters next to each other (I've seen this in other clues).
'a bighead' becomes 'b' (the head letter of 'big').
'onus' after 'b' is 'BONUS'.
Can you help me to learn more?
(Other definitions for bonus that I've seen before include "Additional payment to encourage productivity" , "Extra payment for good results" , "Extra income" , "Additional wage" , "Additional incentive payment" .)