Reckless person in silly hat (6)
I believe the answer is:
madcap
'reckless person' is the definition.
(I've seen this before)
'silly hat' is the wordplay.
'silly' becomes 'mad'.
'hat' becomes 'cap' (I've seen this before).
'mad'+'cap'='MADCAP'
'in' acts as a link.
(Other definitions for madcap that I've seen before include "Reckless, hot-headed" , "Eccentric or reckless person" , "Insane headgear for wildly impulsive person" , "Foolish, reckless" , "Given to reckless behaviour" .)