Fast time, nothing hurt running uphill (7)
I believe the answer is:
ramadan
'fast time' is the definition.
(month in which Muslims fast)
'nothing hurt running uphill' is the wordplay.
'nothing' becomes 'nada' (synonyms).
'hurt running' becomes 'mar' (I can't justify this - if you can you should give a lot more credence to this answer).
'uphill' shows that the letters should be reversed in order (in a down clue, letters go up).
'nada'+'mar'='nadamar'
'nadamar' back-to-front is 'RAMADAN'.
Can you help me to learn more?
(Other definitions for ramadan that I've seen before include "Muslims observe this" , "Moslem fast" , "The penitential ninth month of the Muslim year" , "Religious festival" , "lunar month" .)