In lieu, I have fish (6)
I believe the answer is:
plaice
'fish' is the definition.
(plaice is a kind of fish)
'in lieu i' is the wordplay.
'in' is an insertion indicator.
'lieu' becomes 'place' (synonyms).
'place' placed around 'i' is 'PLAICE'.
'have' acts as a link.
(Other definitions for plaice that I've seen before include "Edible flatfish" , "Important food fish" , "Location of the fish" , "Tasty flatfish I place here" , "Type of fish" .)