Productive fellow to reel it off (7)
I believe the answer is:
fertile
'productive' is the definition.
('fertile' can be a synonym of 'productive')
'fellow to reel it off' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'off' is an anagram indicator.
'reel'+'it'='reelit'
'reelit' anagrammed gives 'ertile'.
'f'+'ertile'='FERTILE'
(Other definitions for fertile that I've seen before include "Fruitful" , "Able to reproduce abundantly" , "Rich in growth" , "Fecund" , "Let rife (anag)" .)