Knock over small tablet (5)
I believe the answer is:
spill
'knock over' is the definition.
(I've seen this in another clue)
'small tablet' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tablet' becomes 'pill' (synonyms).
's'+'pill'='SPILL'
(Other definitions for spill that I've seen before include "Slop; a lighter" , "Cause to fall from horse" , "accident" , "Splatter" , "Firelighter" .)