Liberal encased in concrete? That's lenient! (7)
I believe the answer is:
clement
'lenient' is the definition.
(similar in meaning)
'liberal encased in concrete?' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'encased in' means one lot of letters goes inside another.
'concrete?' becomes 'cement' (cement is a kind of concrete).
'l' inserted within 'cement' is 'CLEMENT'.
'that's' is the link.
(Other definitions for clement that I've seen before include "- - Attlee (PM)" , "Merciful; various Popes" , "Merciful, lenient" , "Pope once" , "opponent for Winston" .)