Agony to trot over the river (7)
I believe the answer is:
torture
'agony' is the definition.
('torture' can be a synonym of 'agony')
'trot over the river' is the wordplay.
'over' says the letters should be written backwards.
'the river' becomes 'ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure').
'trot' reversed gives 'tort'.
'tort'+'ure'='TORTURE'
'to' acts as a link.
(Other definitions for torture that I've seen before include "Inflict pain to extract information" , "cause extreme discomfort?" , "Anguish" , "Agony or suffering" , "Inflict great suffering" .)