Fish I put in position (6)
I believe the answer is:
plaice
'fish' is the definition.
(I know that plaice is a type of fish)
'i put in position' is the wordplay.
'put in' means one lot of letters goes inside another.
'position' becomes 'place' (place is a kind of position).
'i' inserted within 'place' is 'PLAICE'.
(Other definitions for plaice that I've seen before include "Type of fish" , "Important food fish" , "Large flat fish" , "Flatfish; sounds like put" , "Flat fare" .)