Demanding fish dish with oatmeal to start with (9)
I believe the answer is:
gruelling
'demanding' is the definition.
(I've seen this before)
'fish dish with oatmeal to start with' is the wordplay.
'fish dish with' becomes 'ling' (I can't explain this - if you can you should give a lot more credence to this answer).
'oatmeal' becomes 'gruel' (gruel is a kind of oatmeal).
'to start with' means one lot of letters go next to another.
'ling' put after 'gruel' is 'GRUELLING'.
Can you help me to learn more?
(Other definitions for gruelling that I've seen before include "Arduous, backbreaking" , "Severe" , "Demanding" , "Strenuous" .)