On reflection, hard to understand Society drug (5)
I believe the answer is:
speed
'drug' is the definition.
(I know that speed is a type of drug of abuse)
'on reflection hard to understand society' is the wordplay.
'on reflection' is a reversal indicator.
'hard to understand' becomes 'deep' (deep can mean profound or difficult to understand).
'society' becomes 's' (common abbreviation in names of organisations).
'deep'+'s'='deeps'
'deeps' in reverse letter order is 'SPEED'.
(Other definitions for speed that I've seen before include "The going rate" , "Rapidity of movement" , "'More haste, less . . . . .'" , "Alacrity" , "A devilish quick driver?" .)