Fish for a girl on Monday (6)
I believe the answer is:
'fish' is the definition.
(salmon is a kind of fish)
'a girl on monday' is the wordplay.
'a girl' becomes 'salon' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'monday' becomes 'm'.
'salon' enclosing 'm' is 'SALMON'.
'for' is the link.
Can you help me to learn more?
(Other definitions for salmon that I've seen before include "Migratory fish", "Game fish", "Fish (often smoked)", "Food fish; pink shade", "In Gaelic myth, the fish of wisdom".)