Oddball entering without restraint (5)
I believe the answer is:
outre
'oddball' is the definition.
(both can mean eccentric or weird)
'entering without restraint' is the wordplay.
'entering' indicates the answer is hidden within the clue.
'OUTRE' is hidden in the letters of 'without restraint'.
(Other definitions for outre that I've seen before include "Unusual, rather shocking (from French)" , "bizarre style" , "Peculiar" , "Extravagant; bizarre" , "kinky?" .)