Learning to recognise ridge (9)
I believe the answer is:
knowledge
'learning' is the definition.
(thesaurus)
'recognise ridge' is the wordplay.
'recognise' becomes 'know' ('know' can be a synonym of 'recognize').
'ridge' becomes 'ledge' (ledge is a kind of ridge).
'know'+'ledge'='KNOWLEDGE'
'to' is the link.
(Other definitions for knowledge that I've seen before include "Awareness - erudition" , "a small amount is risky" , "Learning, science" , "The result of perception, learning and reasoning" , "Awareness gained through experience" .)