Idea of writing material on fish (7)
I believe the answer is:
inkling
'idea' is the definition.
(inkling is a kind of idea)
'writing material on fish' is the wordplay.
'writing material' becomes 'ink' (substance for writing with).
'on' means one lot of letters go next to another.
'fish' becomes 'ling' (ling is a kind of fish).
'ink'+'ling'='INKLING'
'of' acts as a link.
(Other definitions for inkling that I've seen before include "Vague idea" , "Faint notion" , "Vague notion" , "Suspicion, hunch" , "Intimation" .)