Demanding porridge with fish (9)
I believe the answer is:
gruelling
'demanding' is the definition.
(I've seen this before)
'porridge with fish' is the wordplay.
'porridge' becomes 'gruel' (gruel is a kind of porridge).
'with' says to put letters next to each other.
'fish' becomes 'ling' (ling is a kind of fish).
'gruel'+'ling'='GRUELLING'
(Other definitions for gruelling that I've seen before include "Arduous, backbreaking" , "Demanding" , "Strenuous" , "Severe" .)