Fish for Sally on Monday (6)
I believe the answer is:
salmon
'fish' is the definition.
(salmon is a kind of fish)
'for sally on monday' is the wordplay.
'for sally' becomes 'sal' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'monday' becomes 'Mon' (short for Monday).
'sal'+'mon'='SALMON'
Can you help me to learn more?
(Other definitions for salmon that I've seen before include "Migratory fish" , "Traditionally thought to be the fish of knowledge" , "Fish; pink colour" , "Large fish with pink flesh" , "Pink-fleshed fish" .)