Bird crossing river in horde (5)
I believe the answer is:
drove
'horde' is the definition.
('drove' can be a synonym of 'horde')
'bird crossing river' is the wordplay.
'bird' becomes 'dove' (dove is a kind of bird**).
'crossing' indicates putting letters inside.
'river' becomes 'r'.
'dove' placed around 'r' is 'DROVE'.
'in' is the link.
(Other definitions for drove that I've seen before include "acted like Mr Toad" , "kicked hard" , "Incited" , "Herd of cattle" , "Moving flock; forced" .)