Ageing nurse once looking down on eastern perfume (9)
I believe the answer is:
senescent
'ageing' is the definition.
(I know that ageing can be written as senescent)
'nurse once looking down on eastern perfume' is the wordplay.
'nurse once' becomes 'SEN' (abbreviation for state-enrolled nurse).
'looking down on' means one lot of letters go next to another (in a down clue, some letters go above others).
'eastern' becomes 'e'.
'perfume' becomes 'scent' (synonyms).
'sen'+'e'+'scent'='SENESCENT'
(Other definitions for senescent that I've seen before include "Getting old" , "Showing the deterioration of age" , "Showing age-related deterioration" , "Growing old" , "Ageing" .)