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