Breather following girl’s massive uphill challenge (7)
I believe the answer is:
everest
'massive uphill challenge' is the definition.
'Everest' can be an answer for 'challenge' (Mount Everest, a challenge for mountaineers). I am not sure about the 'massive uphill' bit.
'breather following girl's' is the wordplay.
'breather' becomes 'rest' (a breather is a break or rest).
'following' means one lot of letters go next to another.
'girl' becomes 'eve' ().
'rest' put after 'eve' is 'EVEREST'.
Can you help me to learn more?
(Other definitions for everest that I've seen before include "Tallest mountain ..." , "High mountain" , "Mount, climbed 1953" , "One conquered in 1953" , "Tee revs up the mountain" .)