Preserve line within large family (4)
I believe the answer is:
clan
'large family' is the definition.
(I've seen this before)
'preserve line within' is the wordplay.
'preserve' becomes 'can' (canning is a kind of preserving).
'line' becomes 'l' (used when specifying particular lines from a poem).
'within' indicates putting letters inside.
'can' placed around 'l' is 'CLAN'.
(Other definitions for clan that I've seen before include "One from Benghazi" , "Tribe or collection of families under a cheiftain" , "Large family forming close group, big in Scotland" , "Kin group" , "Group with a tartan?" .)