Massive insect behind a soldier (5)
I believe the answer is:
giant
'massive' is the definition.
(synonyms)
'insect behind a soldier' is the wordplay.
'insect' becomes 'ant' (ant is a kind of insect).
'behind' says to put letters next to each other.
'a soldier' becomes 'GI' (GI can mean a US soldier).
'ant' after 'gi' is 'GIANT'.
(Other definitions for giant that I've seen before include "Colossus" , "A mighty man like Goliath or Carnera" , "Huge person" , "Creature of exceptional size" , "Jumbo" .)