Pass bottom of slope and slip (6)
I believe the answer is:
elapse
'pass' is the definition.
(I know that pass can be written as elapse)
'bottom of slope and slip' is the wordplay.
'bottom of' says to take the final letters.
'and' means one lot of letters go next to another.
'slip' becomes 'lapse' (lapsing is a kind of slipping).
The final letter of 'slope' is 'e'.
'e'+'lapse'='ELAPSE'
(Other definitions for elapse that I've seen before include "Go past" , "Go by - pass away" , "Pass or expire - please" , "Tick by" , "Passing" .)