Fish tricked by weed (7)
I believe the answer is:
haddock
'fish' is the definition.
(I know that haddock is a type of fish)
'tricked by weed' is the wordplay.
'tricked' becomes 'had' (to have someone can mean to trick them).
'by' means one lot of letters go next to another.
'weed' becomes 'dock' (type of weed).
'had'+'dock'='HADDOCK'
(Other definitions for haddock that I've seen before include "Fish like cod but smaller" , "Swimmer" , "White fish" , "Fish similar to but smaller than cod" , "Cod-like fish" .)