Instruments, black in appearance mostly (7)
I believe the answer is:
'instruments' is the definition.
(guitar is a kind of instrument)
'black in appearance mostly' is the wordplay.
'black' becomes 'tar' (I've seen this in other clues).
'in' is an insertion indicator.
'appearance' becomes 'guise' (guise is a kind of appearance).
'mostly' means to remove the last letter (most but not all of the word).
'guise' with its last letter taken away is 'guis'.
'tar' going into 'guis' is 'GUITARS'.
(Other definitions for guitars that I've seen before include "Stringed instruments" , "Axes" .)