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' means one lot of letters goes inside another.
'river' becomes 'aire' (River Aire in Yorkshire).
'f' going into 'aire' is 'AFIRE'.
(Other definitions for afire that I've seen before include "in flames" , "Ablaze" , "Burning" , "Blazing - excited" , "lit up" .)