Extra burden on a bighead (5)
I believe the answer is:
bonus
'extra' is the definition.
(thesaurus)
'burden on a bighead' is the wordplay.
'burden' becomes 'onus' ('onus' can be a synonym of 'burden').
'on' says to put letters next to each other.
'a bighead' becomes 'b' (the head letter of 'big').
'onus' after 'b' is 'BONUS'.
(Other definitions for bonus that I've seen before include "Payment additional to the expected or usual" , "Good-performance payment" , "Additional payment as reward for service" , "Something more than you expected" , "Extra payout" .)