Touch musical instrument all over (4)
I believe the answer is:
abut
'touch' is the definition.
(abutting is a kind of touching)
'musical instrument all over' is the wordplay.
'musical instrument' becomes 'tuba' (tuba is a kind of musical instrument).
'all over' says the letters should be written backwards.
'tuba' backwards is 'ABUT'.
(Other definitions for abut that I've seen before include "Be next door to" , "Lean against the odd tuba" , "Of building or land, end or lean against" , "Lie adjacent to" , "Touch, lean on" .)