Desert marathon perhaps an exhausting grind (3,4)
I believe the answer is:
rat race
'an exhausting grind' is the definition.
(term for working life)
'desert marathon' is the wordplay.
'desert' becomes 'rat' (abandon treacherously).
'marathon' becomes 'race' (I've seen this in another clue).
'rat'+'race'='RAT RACE'
'perhaps' acts as a link.
(Other definitions for rat race that I've seen before include "competitive lifestyle" , "nine-to-five" , "Competitive struggle for wealth or power" , "The struggle for survival" , "Working life's struggle" .)