Something artist may use for colouring fish in study (6)
I believe the answer is:
crayon
'something artist may use for colouring' is the definition.
(crayons may be used to colour things in)
'fish in study' is the wordplay.
'fish' becomes 'ray' (ray is a kind of fish).
'in' means one lot of letters goes inside another.
'study' becomes 'con' (con can informally mean to study).
'ray' put within 'con' is 'CRAYON'.
(Other definitions for crayon that I've seen before include "One colours with this" , "Wax pen" , "drawer" , "alternative to charcoal" , "Stick to colour" .)