Ken's policy expert climbing rock shelf (9)
I believe the answer is:
knowledge
'ken's' is the definition.
(thesaurus)
'policy expert climbing rock shelf' is the wordplay.
'policy expert' becomes 'wonk' (I've seen this in another clue).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'rock shelf' becomes 'ledge' (I've seen this before).
'wonk' back-to-front is 'know'.
'know'+'ledge'='KNOWLEDGE'
(Other definitions for knowledge that I've seen before include "Cognition, wisdom" , "Information; awareness" , "Info" , "An organised body of information" , "practical skill" .)