Unpleasant period having fibre to eat (8)
I believe the answer is:
roughage
'fibre to eat' is the definition.
'unpleasant period' is the wordplay.
'unpleasant' becomes 'rough' ('rough' can be similar in meaning to 'unpleasant').
'period' becomes 'age' (I've seen this before).
'rough'+'age'='ROUGHAGE'
'having' is the link.
(Other definitions for roughage that I've seen before include "Coarse food" , "Essential food" , "Indigestible fibre" , "It aids digestion" , "Vegetable fibre" .)