Seat in small plane? (5)
I believe the answer is:
stool
'seat' is the definition.
(I know that stool is a type of seat)
'small plane?' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'plane?' becomes 'tool' (plane is a kind of tool).
's'+'tool'='STOOL'
'in' is the link.
(Other definitions for stool that I've seen before include "Backless chair" , "Three-legged backless seat" , "Motion" , "Armless seat" , "item of furniture here" .)