Starts to row up River Avon, largely in the countryside (5)
I believe the answer is:
rural
'in the countryside' is the definition.
(I've seen this before)
'starts to row up river avon largely' is the wordplay.
'starts to' says to take the initial letters.
The first letters of 'row up river avon largely' is 'RURAL'.
(Other definitions for rural that I've seen before include "Countryfied" , "Old country fellow" , "outside the city?" , "Not urban" , "Pastoral or agricultural" .)