Leaders of some key industries go downhill rapidly (3)
I believe the answer is:
ski
'go downhill rapidly' is the definition.
(I've seen this in another clue)
'leaders of some key industries' is the wordplay.
'leaders of' says to take the initial letters.
The first letters of 'some key industries' is 'SKI'.
(Other definitions for ski that I've seen before include "Use runners crossing snow" , "Winter-sports item" , "Move over snow on runners" , "Journey" , "Snow blade" .)