Attention, gun is secret service agent's accessory (8)
I believe the answer is:
earpiece
'secret service agent's accessory' is the definition.
(I've seen this in another clue)
'attention gun' is the wordplay.
'attention' becomes 'ear' (having someone's ear is having their attention).
'gun' becomes 'piece' (piece is a kind of gun).
'ear'+'piece'='EARPIECE'
'is' acts as a link.
(Other definitions for earpiece that I've seen before include "box?" , "Through which one may listen" , "receiver" , "Headset part" , "What lets you hear communications" .)