Bill King perhaps supports mountain climbing (7)
I believe the answer is:
placard
'bill' is the definition.
(I've seen this before)
'king perhaps supports mountain climbing' is the wordplay.
'king perhaps' becomes 'card' (king is a kind of card).
'supports' is a charade indicator (letters next to each other) (bottom letters provide a support for the top ones).
'mountain' becomes 'alp' (alp is a kind of mountain**).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'alp' reversed gives 'pla'.
'card' after 'pla' is 'PLACARD'.
(Other definitions for placard that I've seen before include "Slogan or message of protest on board" , "what may be carried in a demo" , "Bill" , "Sign posted in a public place as an advertisement" , "Sign for public display" .)