Sound horn on top of Harley chopper? (5)
I believe the answer is:
tooth
'chopper?' is the definition.
(chopper is a kind of tooth)
'sound horn on top of harley' is the wordplay.
'sound horn' becomes 'toot' (I've seen this before).
'on top of' suggests taking the first letters.
The initial letter of 'harley' is 'h'.
'toot'+'h'='TOOTH'
(Other definitions for tooth that I've seen before include "Gearwheel cog" , "1 across" , "Fang; cog" , "Piece of ivory" , "Bonelike structure" .)