Climbing it, is one apt to breathe badly? (4)
I believe the answer is:
hill
'climbing it is one apt' is the definition.
I can't judge whether this defines the answer.
'breathe badly?' is the wordplay.
'breathe' becomes 'h' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'badly?' becomes 'ill' (eg in ill-advised).
'h'+'ill'='HILL'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for hill that I've seen before include "Canvasser of opinions" , "Box ---, Emma picnic site (J.Austen)" , "Miniature mountain" , "Raised area of land" , "High point, but not a mountain" .)