Adduce Beethoven’s Ninth, say, on reflection, describing variation in form? (7)
I believe the answer is:
genetic
'describing variation in form?' is the definition.
(as in genetic variety)
'adduce beethoven's ninth say on reflection' is the wordplay.
'adduce' becomes 'cite' (synonyms).
'beethoven's ninth' becomes 'n' (9th letter of 'beethoven').
'say' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'on reflection' shows that the letters should be reversed in order.
'cite'+'n'+'eg'='citeneg'
'citeneg' back-to-front is 'GENETIC'.
(Other definitions for genetic that I've seen before include "Pertaining to heredity" , "origin of something" , "coming from a common source" , "(The code) of our DNA" , "relating to origin" .)