Observe the mutt put in a safe (6)
I believe the answer is:
secure
'a safe' is the definition.
(thesaurus)
'observe the mutt put in' is the wordplay.
'observe' becomes 'see' (I've seen this before).
'the mutt' becomes 'cur' ('cur' can be a synonym of 'mutt').
'put in' means one lot of letters goes inside another.
'see' enclosing 'cur' is 'SECURE'.
(Other definitions for secure that I've seen before include "Safe against attack or well fastened" , "properly restrained" , "Seize; make fast" , "Incapable of being tampered with" , "Make safe or fasten" .)