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' is an insertion indicator.
'sees' enclosing 'cur' is 'SECURES'.
'and' is the link.
(Other definitions for secures that I've seen before include "Makes safe through rescues" , "Guarantees" , "Makes firmly attached" , "Closes, fortifies" , "Ties down" .)