Monsters with poisonous gas defeat one (7)
I believe the answer is:
colossi
'monsters' is the definition.
('colossus' can be a synonym of 'monster')
'with poisonous gas defeat one' is the wordplay.
'with' is a charade indicator (letters next to each other).
'poisonous gas' becomes 'co' (I've seen this in other clues).
'defeat' becomes 'loss' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'co' put next to 'loss' is 'coloss'.
'coloss'+'i'='COLOSSI'
(Other definitions for colossi that I've seen before include "Huge statues" , "powerful people" , "Very large statues" , "Gigantic statues" , "huge figures" .)