Go off on the subject of the duo (6)
I believe the answer is:
repair
'go off' is the definition.
'repair' can be an answer for 'go' (I've seen this before). I am not sure about the 'off' bit.
'the subject of the duo' is the wordplay.
'the subject of' becomes 're' ('re' means regarding or about).
'the duo' becomes 'pair' (synonyms).
're'+'pair'='REPAIR'
'on' is the link.
Can you help me to learn more?
(Other definitions for repair that I've seen before include "do some darning" , "Make one's way (to); fix" , "Restore to working condition" , "Once, return" , "Restore (something) to good condition" .)