Key responsibility brings reward (5)
I believe the answer is:
bonus
'reward' is the definition.
(thesaurus)
'key responsibility' is the wordplay.
'key' becomes 'b' (eg B major, musical key).
'responsibility' becomes 'onus' (I've seen this before).
'b'+'onus'='BONUS'
'brings' acts as a link.
(Other definitions for bonus that I've seen before include "Added extra" , "Perk" , "Additional wage" , "Extra money paid on top of salary" , "Additional payment as reward for service" .)