Demanding thin porridge with fish (9)
I believe the answer is:
gruelling
'demanding' is the definition.
(I've seen this before)
'thin porridge with fish' is the wordplay.
'thin porridge' becomes 'gruel' (I've seen this before).
'with' 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 "Severe" , "Demanding" , "Arduous, backbreaking" , "Strenuous" .)