Settle boy holding ring in important part of the cargo (7)
I believe the answer is:
payload
'the cargo' is the definition.
(synonyms)
'settle boy holding ring in important part' is the wordplay.
'settle' becomes 'pay' (paying is a kind of settling).
'boy' becomes 'lad' (informal term for boy).
'holding' is an insertion indicator.
'ring in important part' becomes 'o' (I can't justify this - if you can you should give a lot more credence to this answer).
'lad' placed around 'o' is 'load'.
'pay'+'load'='PAYLOAD'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for payload that I've seen before include "Passengers, cargo or bombs" , "Revenue-earning cargo" , "Cargo, bombs, on e.g. plane" , "Missile's warhead" .)