Quail, being left stuck in seat (6)
I believe the answer is:
blench
'quail' is the definition.
(thesaurus)
'left stuck in seat' is the wordplay.
'left' becomes 'l' (common abbreviation).
'stuck in' means one lot of letters goes inside another.
'seat' becomes 'bench' (bench is a kind of seat**).
'l' placed within 'bench' is 'BLENCH'.
'being' acts as a link.
(Other definitions for blench that I've seen before include "Cringe" , "Make sudden move" , "Shrink back, flinch" , "Turn pale with fear" .)