Several guards in front of gatehouse look cocky (12)
I believe the answer is:
vainglorious
'cocky' is the definition.
(I've seen this before)
'several guards in front of gatehouse look' is the wordplay.
'several' becomes 'various' (synonyms).
'guards' is an insertion indicator (inserted letters are guarded).
'front of' suggests taking the first letters (the letter at the front of the word).
'look' becomes 'lo' (archaic exclamation meaning 'look!').
The first letter of 'gatehouse' is 'g'.
'in'+'g'+'lo'='inglo'
'various' enclosing 'inglo' is 'VAINGLORIOUS'.
(Other definitions for vainglorious that I've seen before include "Excessively self-proud" , "Boastful; rousing viola (anag.)" , "Egotistic" , "Self-important -- savouring oil (anag)" , "Boasting" .)