Informer picked up illegal bit of tack (8)
I believe the answer is:
noseband
'bit of tack' is the definition.
(part of a horse's tack)
'informer picked up illegal' is the wordplay.
'informer' becomes 'nose' (I've seen this before).
'picked up' indicates a 'sounds like' (homophone) clue.
'illegal' becomes 'banned' (similar in meaning).
'banned' sounds like 'band'.
'nose'+'band'='NOSEBAND'
(Other definitions for noseband that I've seen before include "Piece of tack" , "bridle strap" .)