Enormous US soldier initially greeting worker in charge (8)
I believe the answer is:
gigantic
'enormous' is the definition.
(synonyms)
'us soldier initially greeting worker in charge' is the wordplay.
'us soldier' becomes 'GI' (GI can mean an American soldier).
'initially' suggests taking the first letters.
'worker' becomes 'ant' (worker is a type of ant).
'in charge' becomes 'ic' (abbreviation for 'in charge').
The first letter of 'greeting' is 'g'.
'gi'+'g'+'ant'+'ic'='GIGANTIC'
(Other definitions for gigantic that I've seen before include "Fantastically great" , "walloping" , "'Enormous, mammoth (8)'" , "Thumping" , "Exceedingly large" .)