Stupid, small lily arrangement (5)
I believe the answer is:
silly
'stupid' is the definition.
(I've seen this before)
'small lily arrangement' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'arrangement' indicates anagramming the letters.
'lily' with letters rearranged gives 'illy'.
's'+'illy'='SILLY'
(Other definitions for silly that I've seen before include "Daft" , "Ridiculous, frivolous" , "Stupid" , "Preposterous" , "Absurd" .)