Illumination required by area for landing (9)
I believe the answer is:
alighting
'landing' is the definition.
(alighting is a kind of landing)
'illumination required by area' is the wordplay.
'illumination' becomes 'lighting' (lighting is a kind of illumination).
'required by' means one lot of letters go next to another.
'area' becomes 'a' (maths abbreviation).
'lighting' put after 'a' is 'ALIGHTING'.
'for' is the link.
(Other definitions for alighting that I've seen before include "Descending from a train, bus, etc." , "Getting off" , "Landing" , "coming" , "Settling" .)