Comfortable with weapon (4)
I believe the answer is:
warm
'comfortable' is the definition.
(at a comfortable temperature)
'with weapon' is the wordplay.
'with' becomes 'w' (abbreviation).
'weapon' becomes 'arm' (synonyms).
'w'+'arm'='WARM'
(Other definitions for warm that I've seen before include "Lively" , "Cordial" , "Animated" , "Heated; enthusiastic" , "On the brink of making discovery" .)