Children swallowing soft drug (5)
I believe the answer is:
speed
'drug' is the definition.
(speed is a kind of drug)
'children swallowing soft' is the wordplay.
'children' becomes 'seed' (I've seen this before).
'swallowing' is an insertion indicator.
'soft' becomes 'p' (musical abbreviation for 'piano').
'seed' enclosing 'p' is 'SPEED'.
(Other definitions for speed that I've seen before include "Alacrity" , "make rapid progress" , "Swiftness" , "Dispatch" , "Rapidity of movement" .)