For instance, possibly all out to be legitimate (5)
I believe the answer is:
legal
'be legitimate' is the definition.
('legal' can be similar in meaning to 'legitimate')
'for instance possibly all out' is the wordplay.
'for instance' becomes 'eg' ('e.g.' can be a synonym of 'for instance').
'possibly' indicates an anagram.
'out' indicates putting letters inside.
'all' anagrammed gives 'lal'.
'eg' put within 'lal' is 'LEGAL'.
'to' is the link.
(Other definitions for legal that I've seen before include "within the rules" , "right" , "Permitted by law" , "Authorised" , "Compliant with the law" .)