Cat to lock soldier back in (7)
I believe the answer is:
tigress
'cat' is the definition.
(tigress is a kind of cat)
'lock soldier back in' is the wordplay.
'lock' becomes 'tress' (I've seen this before).
'soldier' becomes 'gi' (government issue or general issue).
'back' is a reversal indicator.
'in' means one lot of letters goes inside another.
'gi' written backwards gives 'ig'.
'tress' enclosing 'ig' is 'TIGRESS'.
'to' acts as a link.
(Other definitions for tigress that I've seen before include "Large she-cat" , "Female big cat" , "Ferocious female" , "Large female cat" , "hunting animal" .)