Relative, naive, acted oddly (5)
I believe the answer is:
niece
'relative' is the definition.
(niece is a kind of relative)
'naive acted oddly' is the wordplay.
'oddly' means one should take alternating letters.
'naive'+'acted'='naiveacted'
The alternate letters of 'naiveacted' are 'NIECE'.
(Other definitions for niece that I've seen before include "Young female relative" , "Criminal" , "Kinswoman" , "A sibling's daughter" , "Female relation" .)