River craft have no British cereal (4)
I believe the answer is:
oats
'cereal' is the definition.
(I've seen this in another clue)
'river craft have no british' is the wordplay.
'river craft' becomes 'boats' (craft used on a river).
'have no' suggests deleting specific letters.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'boats' with 'b' taken out is 'OATS'.
(Other definitions for oats that I've seen before include "Sow wild ones in disreputable youth" , "Edible grass seeds" , "Cereal food for horses" , "Grain for porridge" , "Hardy cereals eaten by horses" .)