Something ventured climbing a peak (4)
I believe the answer is:
etna
'a peak' is the definition.
(I have seen 'high peak' mean 'etna' so perhaps 'peak' could also mean 'etna')
'something ventured climbing' is the wordplay.
'something ventured' becomes 'ante' (money staked in a card game).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'ante' back-to-front is 'ETNA'.
(Other definitions for etna that I've seen before include "Explosive type" , "Threatening mountain" , "Highest peak in Italy south of the Alps" , "Impressive mount" , "Sicilian volcano" .)