Farm animal to lower head in river bend (5)
I believe the answer is:
oxbow
'river bend' is the definition.
(an oxbow is a bend in a river)
'farm animal to lower head' is the wordplay.
'farm animal' becomes 'ox'.
'to lower head' becomes 'bow' (bow one's head).
'ox'+'bow'='OXBOW'
'in' is the link.
(Other definitions for oxbow that I've seen before include "Loop formed by a horseshoe bend in a river" , "State of panic or fear" , "River-loop lake" , "water feature" , "Lake formed by a bend in a river" .)