Keep line in order (6)
I believe the answer is:
castle
'keep' is the definition.
(I've seen this before)
'line in order' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' indicates putting letters inside.
'order' becomes 'caste' (type of social rank or order).
'l' placed inside 'caste' is 'CASTLE'.
(Other definitions for castle that I've seen before include "Rook, on chess board" , "Imposing residence, perhaps fortified" , "Fort" , "a piece in the corner" , "Lincoln, Ludlow or Leeds" .)