Deceit cut cocaine supply for mathematician (9)
I believe the answer is:
fibonacci
'mathematician' is the definition.
(I've seen this before)
'deceit cut cocaine supply' is the wordplay.
'deceit' becomes 'fib' (I can't justify this - if you can you should give a lot more credence to this answer).
'cut' means to remove the last letter.
'supply' indicates anagramming the letters (in a supple way).
'cocaine' with its last letter taken away is 'cocain'.
'cocain' with letters rearranged gives 'onacci'.
'fib'+'onacci'='FIBONACCI'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for fibonacci that I've seen before include "Italian mathematician who gave name to sequence" , "Mathematician giving name to a series" , "Maths whiz" , "Thirteenth century Italian mathematician associated with series one, one, two, three, five, eight etc" .)