Traveller's fighting to save variable travel expense (8)
I believe the answer is:
wayfarer
'traveller's' is the definition.
(wayfarer is a kind of traveller)
'fighting to save variable travel expense' is the wordplay.
'fighting' becomes 'war'.
'to save' is an insertion indicator.
'variable' becomes 'y' (algebra).
'travel expense' becomes 'fare' (I've seen this in another clue).
'y'+'fare'='yfare'
'war' placed around 'yfare' is 'WAYFARER'.
(Other definitions for wayfarer that I've seen before include "Person who travels on foot" , "Trekker" , "One walking" , "One journeying on foot" , "Foot-traveller" .)