A rewarding burden (7)
I believe the answer is:
payload
'a rewarding burden' is the definition.
'payload' can be an answer for 'burden' (thesaurus). I'm not sure about the 'a rewarding' bit.
'rewarding burden' is the wordplay.
'rewarding' becomes 'pay' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'burden' becomes 'load' (synonyms).
'pay'+'load'='PAYLOAD'
Can you help me to learn more?
(Other definitions for payload that I've seen before include "Missile's warhead" , "Revenue-earning cargo" , "Passengers, cargo or bombs" , "Cargo, bombs, on e.g. plane" .)