All the way to the summit sounded flatter on paper (5,2)
I believe the answer is:
write up
'all' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'way to the summit sounded flatter on paper' is the wordplay.
I cannot quite see how this works, but
'way' could be 'w' (Compass direction - West) and 'w' is present in the answer.
'paper' could be 'i' (UK newspaper) and 'i' is present in the answer.
'to the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'on' could be 're' ('on' can be mean 'with reference to') and 're' is present in the remaining letters.
The remaining letters 'up' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'the' is the link.
Can you help me to learn more?
(Other definitions for write up that I've seen before include "Published account" , "report" .)