Look back and note source, one that is retained (9)
I believe the answer is:
reminisce
'look back' is the definition.
(I've seen this before)
'note source one that is retained' is the wordplay.
'note' becomes 're'.
'source one' becomes 'mince' (I can't justify this - if you can you should give a lot more credence to this answer).
'that is' becomes 'is'.
'retained' means one lot of letters goes inside another (inserted letters are held or retained).
'mince' enclosing 'is' is 'minisce'.
're'+'minisce'='REMINISCE'
'and' is the link.
(Other definitions for reminisce that I've seen before include "reflect" , "Look back at events" , "Say what one remembers" , "Recount memories" , "Think back fondly" .)