Colossal insect chasing soldier (5)
I believe the answer is:
giant
'colossal' is the definition.
(I know that colossal can be written as giant)
'insect chasing soldier' is the wordplay.
'insect' becomes 'ant' (ant is a kind of insect).
'chasing' means one lot of letters go next to another (some letters chase after others).
'soldier' becomes 'gi' (GI can mean a US soldier).
'ant' put after 'gi' is 'GIANT'.
(Other definitions for giant that I've seen before include "Extremely large (person)" , "Very big chap" , "Large mythical figure" , "Colossus" , "Gargantuan" .)