Tramps uncovered organ stop (4)
I believe the answer is:
oboe
'organ stop' is the definition.
(I've seen this in another clue)
'tramps uncovered' is the wordplay.
'tramps' becomes 'hoboes' ('hobo' can be a synonym of 'tramp').
'uncovered' indicates the central letters.
The centre of 'hoboes' is 'OBOE'.
(Other definitions for oboe that I've seen before include "Cor anglais in its family" , "Player" , "Woodwind instrument of treble pitch" , "Source of music" , "source of notes" .)