Continual struggle in desert heat (3,4)
I believe the answer is:
rat race
'continual struggle' is the definition.
'rat race' can be an answer for 'struggle' (I have seen 'Daily struggle ' mean 'rat race' so perhaps 'struggle' could also mean 'rat race'). I'm not sure about the 'continual' bit.
'desert heat' is the wordplay.
'desert' becomes 'rat' (ratting is a kind of deserting).
'heat' becomes 'race' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rat'+'race'='RAT RACE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for rat race that I've seen before include "Desert people in" , "Continual routine of highly competitive activity" , "Grim struggle" , "Fierce competition to succeed in life" , "The daily grind" .)