Unnecessary seats in line to be shifted (11)
I believe the answer is:
inessential
'unnecessary' is the definition.
('inessential' can be similar in meaning to 'unnecessary')
'seats in line to be shifted' is the wordplay.
'to be shifted' indicates anagramming the letters.
'seats'+'in'+'line'='seatsinline'
'seatsinline' is an anagram of 'INESSENTIAL'.
(Other definitions for inessential that I've seen before include "Not absolutely necessary" , "dispensable" , "Unnecessary" , "Not totally necessary" , "Something that's only nice to have" .)