Rest recovering from cold, once across valleys (10)
I believe the answer is:
convalesce
'rest recovering' is the definition.
I can't judge whether this defines the answer.
'cold once across valleys' is the wordplay.
'cold' becomes 'c' (eg on taps).
'across' is an insertion indicator.
'valleys' becomes 'vales' ('vale' can be a synonym of 'valley').
'once' enclosing 'vales' is 'onvalesce'.
'c'+'onvalesce'='CONVALESCE'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for convalesce that I've seen before include "Recover after an illness" , "Improve" .)