Retired city keyboard player, not a mountaineer (8)
I believe the answer is:
alpinist
'mountaineer' is the definition.
(alpinist is a kind of mountaineer)
'retired city keyboard player not a' is the wordplay.
'retired' is a reversal indicator.
'city' becomes 'LA' (Los Angeles).
'keyboard player' becomes 'pianist' (a piano has a keyboard).
'not' is a deletion indicator.
'la' backwards is 'al'.
'pianist' with 'a' removed is 'pinist'.
'al'+'pinist'='ALPINIST'
(Other definitions for alpinist that I've seen before include "Maker of sharp climb" , "Climber of big mountains" , "mountaineer" , "one climbs" .)