Approximate time to produce fibre (8)
I believe the answer is:
roughage
'fibre' is the definition.
(I've seen this before)
'approximate time' is the wordplay.
'approximate' becomes 'rough' (I've seen this before).
'time' becomes 'age' (age is a kind of time).
'rough'+'age'='ROUGHAGE'
'to produce' acts as a link.
(Other definitions for roughage that I've seen before include "Vegetable fibre" , "Bulk" , "It aids digestion" , "Dietary fibre" , "Coarse food" .)