Burning fuel's first put in river (5)
I believe the answer is:
afire
'burning' is the definition.
(on fire)
'fuel's first put in river' is the wordplay.
'fuel's first' becomes 'f' (1st letter of 'fuel').
'put in' indicates putting letters inside.
'river' becomes 'aire' (River Aire in Yorkshire).
'f' inserted into 'aire' is 'AFIRE'.
(Other definitions for afire that I've seen before include "in flames" , "Ablaze" , "Burning" , "Blazing - excited" , "lit up" .)