Fat American soldier bachelor knocked up (3)
I believe the answer is:
big
'fat' is the definition.
(thesaurus)
'american soldier bachelor knocked up' is the wordplay.
'american soldier' becomes 'GI' (informal term for a US soldier).
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'knocked up' says the letters should be written backwards.
'gi'+'b'='gib'
'gib' in reverse letter order is 'BIG'.
(Other definitions for big that I've seen before include "Huge" , "London bell" , "Large; generous" , "oversized" , "Circus tent" .)