Opportunely gathering trendy material for a coat? (5)
I believe the answer is:
paint
'material for a coat?' is the definition.
(coat of paint)
'opportunely gathering trendy' is the wordplay.
'opportunely' becomes 'pat' (pat can mean conveniently or opportunely).
'gathering' indicates putting letters inside (gather can mean to acquire or bring in).
'trendy' becomes 'in' ('in' can mean fashionable).
'pat' enclosing 'in' is 'PAINT'.
(Other definitions for paint that I've seen before include "Apply a coat of colour" , "Eg, emulsion or gloss" , "Daub" , "Pigment" , "Use a brush on canvas" .)