Sound made by donkey — or horse crossing river? (4)
I believe the answer is:
bray
'sound made by donkey' is the definition.
(bray can mean a donkey's sound)
'horse crossing river?' is the wordplay.
'horse' becomes 'bay' (bay is a kind of horse).
'crossing' means one lot of letters goes inside another.
'river?' becomes 'r' (abbreviation for river).
'bay' placed around 'r' is 'BRAY'.
'or' acts as a link.
(Other definitions for bray that I've seen before include "Berkshire village with a 16th-century vicar celebrated in legend and song" , "Donkey's noise" , "Berkshire village with a long-serving, politically flexible 18th-century vicar" , "Cry like a donkey" , "Ass's cry" .)