Understands the dog's in and locks up (7)
I believe the answer is:
secures
'locks up' is the definition.
(I've seen this in another clue)
'understands the dog's in' is the wordplay.
'understands' becomes 'sees' ('see' can be a synonym of 'understand').
'the dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'in' indicates putting letters inside.
'sees' placed around 'cur' is 'SECURES'.
'and' acts as a link.
(Other definitions for secures that I've seen before include "Makes safe through rescues" , "Guarantees" , "Makes firmly attached" , "Closes, fortifies" , "Ties down" .)