Small nut, mostly pink (6)
I believe the answer is:
salmon
'pink' is the definition.
(salmon pink is a kind of pink)
'small nut mostly' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'nut' becomes 'almond' (almond is a type of nut).
'mostly' means to remove the last letter (most but not all of the word).
'almond' with its final letter removed is 'almon'.
's'+'almon'='SALMON'
(Other definitions for salmon that I've seen before include "Oddly, Mal's no fish" , "Food fish; pink shade" , "Underwater swimmers" , "Ol' man's keen on the fish (6)" , "'No alms, oddly, for this fish (6)'" .)