Slipped on small cover (4)
I believe the answer is:
slid
'slipped' is the definition.
('slide' can be a synonym of 'slip')
'on small cover' is the wordplay.
'on' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cover' becomes 'lid' (lid is a kind of cover).
's' next to 'lid' is 'SLID'.
(Other definitions for slid that I've seen before include "Skidded, slewed" , "Moved smoothly along a surface" , "Moved (eg, on ice)" , "Slipped" .)