Peak of affair -- one might blow it (10)
I believe the answer is:
matterhorn
'peak' is the definition.
(Matterhorn is an example)
'affair one might blow it' is the wordplay.
I cannot really see how this works, but
'affair' could be 'matter' ('matter' can be a synonym of 'affair') and 'matter' is present in the answer.
The remaining letters 'horn' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for matterhorn that I've seen before include "Alpine mountain" , "Alpine challenge" , "one of a range" , "Mountain in the Alps" , "A scale that may be difficult" .)