Exclude from French sandbank (5)
I believe the answer is:
debar
'exclude' is the definition.
('debar' can be a synonym of 'exclude')
'from french sandbank' is the wordplay.
'from french' becomes 'de' (French for 'from').
'sandbank' becomes 'bar' (I've seen this before).
'de'+'bar'='DEBAR'
(Other definitions for debar that I've seen before include "Rule out" , "Stop" , "Block" , "Refuse to admit" , "Exclude (from right)" .)