Posh university climber association (3,6)
I believe the answer is:
ivy league
'posh university' is the definition.
The definition suggests a singular noun which matches the answer.
'climber association' is the wordplay.
'climber' becomes 'ivy' (ivy is a climbing plant).
'association' becomes 'league' (league is a kind of association).
'ivy'+'league'='IVY LEAGUE'
Can you help me to learn more?
(Other definitions for ivy league that I've seen before include "Group of US elite universities" , "Brown, Columbia, Cornell, Dartmouth, Harvard, Pennsylvania, Princeton and Yale" , "Top US universities" , "Group of prominent US universities" , "Brown is one of its members" .)