Listening devices a lawman perfects (9)
I believe the answer is:
earphones
'listening devices' is the definition.
(I've seen this before)
'a lawman perfects' is the wordplay.
'a lawman' becomes 'earp' (I've seen this before).
'perfects' becomes 'hones' ('hone' can be a synonym of 'perfect').
'earp'+'hones'='EARPHONES'
(Other definitions for earphones that I've seen before include "listening devices" , "Listening device" , "One phrase (anag.)" .)