Frenchman on donkey in horde (4)
I believe the answer is:
mass
'horde' is the definition.
(I've seen this in another clue)
'frenchman on donkey' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'donkey' becomes 'ass' (ass is a term for a donkey).
'm'+'ass'='MASS'
'in' acts as a link.
(Other definitions for mass that I've seen before include "Body of matter" , "Lump of material" , "Total" , "congregation for > service" , "Roman Catholic service" .)