Aging rose starts off with an extremely nice smell (9)
I believe the answer is:
senescent
'aging' is the definition.
(I know that aging can be written as senescent)
'with an extremely nice smell' is the wordplay.
'with' is a charade indicator (letters next to each other).
'an extremely' becomes 'sene' (I can't justify this - if you can you should believe this answer much more).
'nice smell' becomes 'scent'.
'sene' next to 'scent' is 'SENESCENT'.
'rose starts off' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for senescent that I've seen before include "Showing the deterioration of age" , "Ageing" , "Showing age-related deterioration" , "Growing old" , "Getting old" .)