Big Leo has broken out and is at large (5)
I believe the answer is:
loose
'is at large' is the definition.
(synonyms)
'big leo has broken out' is the wordplay.
'big' becomes 'os'.
'has broken' indicates an anagram (I've seen this in other clues).
'out' indicates putting letters inside.
'leo' is an anagram of 'loe'.
'os' going within 'loe' is 'LOOSE'.
'and' acts as a link.
(Other definitions for loose that I've seen before include "Unsecured; dissolute" , "Hanging" , "Not tightly fixed" , "At liberty" , "Slack or immoral" .)