Dress lacking fine diamond (4)
I believe the answer is:
rock
'diamond' is the definition.
(rock can informally mean a diamond)
'dress lacking fine' is the wordplay.
'dress' becomes 'frock' (a frock is a fancy dress).
'lacking' is a deletion indicator.
'fine' becomes 'f' (abbreviation in pencil classifications).
'frock' with 'f' removed is 'ROCK'.
(Other definitions for rock that I've seen before include "Large stone - type of music" , "See 17" , "Swing" , "Be unstable; lull (baby)" , "musical genre?" .)