When to live outside is reprehensible (4)
I believe the answer is:
base
'reprehensible' is the definition.
(thesaurus)
'when to live outside' is the wordplay.
'when' becomes 'as'.
'to live' becomes 'be' ('be' can be a synonym of 'live').
'outside' indicates putting letters inside.
'as' going within 'be' is 'BASE'.
'is' is the link.
(Other definitions for base that I've seen before include "'Foundation, bottom (4)'" , "Foot; ignoble" , "Reprobate" , "Foundation; counterfeit" , "Venial; headquarters" .)