About to be suspicious of the defence (7)
I believe the answer is:
redoubt
'the defence' is the definition.
(redoubt is a kind of defence)
'about to be suspicious of' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to be suspicious of' becomes 'doubt'.
're'+'doubt'='REDOUBT'
(Other definitions for redoubt that I've seen before include "Entrenched stronghold" , "Fortified stronghold" , "Keep losing confidence in" , "Tube rod (anag) - fortification" , "Obtrude like outer fortification" .)