Central Line stops bland (4)
I believe the answer is:
mild
'bland' is the definition.
(I've seen this in another clue)
'central line stops' is the wordplay.
'central' becomes ''mid' (I've seen this in another clue).
'line' becomes 'l' (used when specifying particular lines from a poem).
'stops' means one lot of letters goes inside another (to stop can mean to block a gap).
'mid' going around 'l' is 'MILD'.
(Other definitions for mild that I've seen before include "Not severe or harsh" , "old cardinals ..." , "Far from extreme" , "Moderately warm - type of beer" , "Gentle in temper and disposition" .)