Fierce female to lock soldier back inside (7)
I believe the answer is:
tigress
'fierce female' is the definition.
(I've seen this in another clue)
'lock soldier back inside' is the wordplay.
'lock' becomes 'tress' (both can mean a piece of hair).
'soldier' becomes 'GI' (term for US soldier).
'back' says the letters should be written in reverse.
'inside' means one lot of letters goes inside another.
'gi' backwards is 'ig'.
'tress' enclosing 'ig' is 'TIGRESS'.
'to' acts as a link.
(Other definitions for tigress that I've seen before include "Female striped cat" , "Wild woman" , "Fierce female" , "Wild animal" , "Passionate woman" .)