Put a line in position for a fish (6)
I believe the answer is:
plaice
'a fish' is the definition.
(I know that plaice is a type of fish)
'put a line in position' is the wordplay.
'put a line' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'position' becomes 'place' (synonyms).
'i' put inside 'place' is 'PLAICE'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for plaice that I've seen before include "Important food fish" , "Location of the fish" , "Flatfish; sounds like put" , "Edible flatfish" , "Tasty flatfish I place here" .)