Contagious enthusiasm for endless high temperatures (6)
I believe the answer is:
fevers
'high temperatures' is the definition.
(I've seen this in another clue)
'contagious enthusiasm for endless' is the wordplay.
'contagious enthusiasm' becomes 'fever' (I've seen this in another clue).
'for' is a charade indicator (letters next to each other).
'endless' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fever'+'s'='FEVERS'
Can you help me to learn more?
(Other definitions for fevers that I've seen before include "Diseases marked by great bodily heat" , "Abnormal rises in body temperature" .)