Considers sounding out routes to the summit (6,2)
I believe the answer is:
weighs up
'routes to the summit' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'considers sounding' is the wordplay.
'considers' becomes 'weighs' ('weigh' can be a synonym of 'consider').
'sounding' becomes 'up' (I can't explain this - if you can you should give a lot more credence to this answer).
'weighs'+'up'='WEIGHS UP'
'out' is the link.
Can you help me to learn more?
(Another definition for weighs up that I've seen is " judges".)