Lock restrains one grand cat (7)
I believe the answer is:
tigress
'cat' is the definition.
(tigress is a kind of cat)
'lock restrains one grand' is the wordplay.
'lock' becomes 'tress' (I've seen this before).
'restrains' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'i'+'g'='ig'
'tress' enclosing 'ig' is 'TIGRESS'.
(Other definitions for tigress that I've seen before include "Female striped cat" , "hunting animal" , "A beastly female" , "Female wildcat" , "Wild animal" .)