Climbs, but when it gets cold, stops (7)
I believe the answer is:
ascends
'climbs' is the definition.
(I've seen this in another clue)
'when it gets cold stops' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'it' becomes 'send' (I can't justify this - if you can you should believe this answer much more).
'gets' indicates putting letters inside (gets can mean captures or absorbs).
'cold' becomes 'C' (eg on taps).
'stops' indicates putting letters inside (to stop can mean to block a gap).
'send' enclosing 'c' is 'scend'.
'as' going around 'scend' is 'ASCENDS'.
'but' is the link.
Can you help me to learn more?
(Other definitions for ascends that I've seen before include "Travels upwards" , "Climbs, mounts" , "Rises" , "Moves upwards" .)