Feline to lock soldier back inside (7)
I believe the answer is:
tigress
'feline' is the definition.
(tigress is a kind of feline)
'lock soldier back inside' is the wordplay.
'lock' becomes 'tress' (both can mean a piece of hair).
'soldier' becomes 'GI' (GI can mean a US soldier).
'back' is a reversal indicator.
'inside' indicates putting letters inside.
'gi' backwards is 'ig'.
'tress' going around 'ig' is 'TIGRESS'.
'to' acts as a link.
(Other definitions for tigress that I've seen before include "Striped feline female" , "Wild cat (female)" , "Wild animal" , "Large she-cat" , "Striped female animal" .)