Beset by trouble, airmen with ultimately duff fuel (8)
I believe the answer is:
paraffin
'fuel' is the definition.
(paraffin is a kind of fuel)
'beset by trouble airmen with ultimately duff' is the wordplay.
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'trouble' becomes 'pain' ('pain' can be a synonym of 'trouble'**).
'airmen' becomes 'raf'.
'with' indicates putting letters inside.
'ultimately' says to take the final letters.
The last letter of 'duff' is 'f'.
'raf' placed around 'f' is 'raff'.
'pain' going around 'raff' is 'PARAFFIN'.
(Other definitions for paraffin that I've seen before include "Waxy flammable substance" , "Flammable wax" , "Petroleum product" , "Liquid fuel" , "Kerosine" .)