Deception about a Trojan Horse's cover (9)
I believe the answer is:
caparison
'horse's cover' is the definition.
(I've seen this before)
'deception about a trojan' is the wordplay.
'deception' becomes 'con' (I've seen this before).
'about' is an insertion indicator.
'trojan' becomes 'paris' (I've seen this before).
'a'+'paris'='aparis'
'con' placed around 'aparis' is 'CAPARISON'.
(Other definitions for caparison that I've seen before include "Ornamental covering over a horse's saddle or harness" , "Rich clothing" , "Cover for a horse" , "horse's cover" , "covering for the Shires?" .)