Reject material upsetting the French (5)
I believe the answer is:
repel
'reject' is the definition.
(repelling is a kind of rejecting)
'material upsetting the french' is the wordplay.
'material' becomes 'rep' (rep is a kind of material).
'upsetting' says the letters should be written backwards.
'the french' becomes 'le' ('the' in French).
'le' backwards is 'el'.
'rep'+'el'='REPEL'
(Other definitions for repel that I've seen before include "Drive back, rebuff" , "Drive or force back" , "Fill with distaste" , "Successfully resist" , "Drive away like leper" .)