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' means one lot of letters goes inside another.
'lieu' becomes 'place' (synonyms).
'place' enclosing 'i' is 'PLAICE'.
'have' is the 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" .)