On reflection, second prize is pants (7)
I believe the answer is:
drawers
'is pants' is the definition.
(underpants)
'on reflection second prize' is the wordplay.
'on reflection' is a reversal indicator.
'second' becomes 's' (common abbreviation).
'prize' becomes 'reward' (a prize may be given as a reward).
's'+'reward'='sreward'
'sreward' reversed gives 'DRAWERS'.
(Other definitions for drawers that I've seen before include "Could be a chest of them but they're worn lower down" , "pants" , "undergarment" , "designers" , "Underwear (for artists?)" .)