Shock when pair climbing spotted mount (9)
I believe the answer is:
appaloosa
'mount' is the definition.
(I know that Appaloosa is a type of mount)
'shock when pair climbing' is the wordplay.
'shock' becomes 'appal' (synonyms).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'pair' becomes 'oo' (resembles 00, a pair of ducks in cricket).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'as'+'oo'='asoo'
'asoo' back-to-front is 'oosa'.
'appal'+'oosa'='APPALOOSA'
'spotted' is the link.
(Another definition for appaloosa that I've seen is " Horse breed, with spotted rump".)