Drop small piece of sugar (5)
I believe the answer is:
slump
'drop' is the definition.
(slumping is a kind of dropping)
'small piece of sugar' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'piece of sugar' becomes 'lump' (I've seen this before).
's'+'lump'='SLUMP'
(Other definitions for slump that I've seen before include "Marked decline" , "Sudden market drop" , "Commercial decline" , "Sudden drop in business" , "Bad economic recession" .)