Fish for a girl on Monday (6)
I believe the answer is:
salmon
'fish' is the definition.
(I know that salmon is a type of fish)
'girl on monday' is the wordplay.
'girl' becomes 'sal' (feminine given name).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'monday' becomes 'Mon' (short for Monday).
'sal'+'mon'='SALMON'
'for a' is the link.
(Other definitions for salmon that I've seen before include "Large fish with pink flesh" , "Food fish; pink shade" , "Ol' man's keen on the fish (6)" , "pink-orange in colour" , "Parr or grilse" .)