Fish had to go in front of weed (7)
I believe the answer is:
haddock
'fish' is the definition.
(haddock is a kind of fish)
'had to go in front of weed' is the wordplay.
'to go in front of' is a charade indicator (letters next to each other).
'weed' becomes 'dock' (type of weed).
'had'+'dock'='HADDOCK'
(Other definitions for haddock that I've seen before include "Fish similar to but smaller than cod" , "Cod-like fish" , "Edible fish like small cod" , "Swimmer" , "Popular food fish" .)