Unemotional female soldier breaks free (6)
I believe the answer is:
frigid
'unemotional' is the definition.
(thesaurus)
'female soldier breaks free' is the wordplay.
'female' becomes 'f' (common abbreviation).
'soldier' becomes 'GI' (GI can mean a US soldier).
'breaks' is an insertion indicator (inserted letters break into the word).
'free' becomes 'rid' (to be free of something is to be rid of it).
'gi' inserted within 'rid' is 'rigid'.
'f'+'rigid'='FRIGID'
(Other definitions for frigid that I've seen before include "far from warm" , "Glacial" , "Lacking affection or warmth" , "Sexually unresponsive" , "Passionless, unresponsive" .)