What one may hear from horse thief (6)
I believe the answer is:
nicker
I believe this is a double definition.
'what one may hear from horse' is the first definition.
(noise made by a horse)
'thief' is the second definition.
(I've seen this before)
(Other definitions for nicker that I've seen before include "thief" , "One steals" , "GBP1 (slang)" , "PS1 (slang)" , "Money" .)