Head off as an animal doctor, crossing river (5)
I believe the answer is:
avert
'head off' is the definition.
('avert' can be a synonym of 'head off')
'an animal doctor crossing river' is the wordplay.
'an' becomes 'a'.
'animal doctor' becomes 'vet' (I've seen this before).
'crossing' means one lot of letters goes inside another.
'river' becomes 'r'.
'vet' going around 'r' is 'vert'.
'a'+'vert'='AVERT'
'as' acts as a link.
(Other definitions for avert that I've seen before include "Ward off, deflect" , "Turn away from, say gaze" , "Turn aside or ward off" , "Head off" , "Turn aside, prevent" .)