Naive decision that hurt (6)
I believe the answer is:
callow
'naive' is the definition.
(synonyms)
'decision that hurt' is the wordplay.
'decision' becomes 'call' (call is a kind of decision).
'that hurt' becomes 'ow' (exclamation of pain).
'call'+'ow'='CALLOW'
(Other definitions for callow that I've seen before include "Simon ---, actor; immature" , "Immature and inexperienced, like youth" , "Young and inexperienced" , "Inexperienced or unsophisticated, as a youth say" , "Jejune" .)