Pigs or snakes crossing river (5)
I believe the answer is:
boars
'pigs' is the definition.
(male pigs)
'snakes crossing river' is the wordplay.
'snakes' becomes 'boas' (boa is a kind of snake).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation).
'boas' going around 'r' is 'BOARS'.
'or' acts as a link.
(Other definitions for boars that I've seen before include "Male badgers" , "Wild swine" , "Male or wild pigs" .)