Airliner emptied, a line appearing under drill on a plane, say (8)
I believe the answer is:
arboreal
'on a plane say' is the definition.
(relating to trees - plane is a type of tree)
'airliner emptied a line appearing under drill' is the wordplay.
'emptied' suggests removing the centre.
'line' becomes 'l' (used when specifying particular lines from a poem).
'appearing under' means one lot of letters go next to another (in a down clue, some letters go under others).
'drill' becomes 'bore' (synonyms).
'airliner' with its centre taken out is 'ar'.
'a'+'l'='al'
'al' after 'bore' is 'boreal'.
'ar'+'boreal'='ARBOREAL'
(Other definitions for arboreal that I've seen before include "Like the sloth" , "found in branches" , "Pertaining to trees" , "Resembling a tree" , "Inhabiting trees" .)