Almost miss promotion that could help ascending the slippery slope (3,4)
I believe the answer is:
ski lift
'that could help ascending the slippery slope' is the definition.
(machine carrying people up a ski slope)
'almost miss promotion' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'miss' becomes 'skip' (skipping is a kind of missing).
'promotion' becomes 'lift' (a promotion lifts you up).
'skip' with its final letter removed is 'ski'.
'ski'+'lift'='SKI-LIFT'
(Other definitions for ski lift that I've seen before include "It can help you get up at Aviemore" , "Way to the top" , "Device for transporting people to top of piste" , "one can ride up here" , "Device for carrying skiers up a slope" .)