Desire often extremely unrestrained (6)
I believe the answer is:
wanton
'unrestrained' is the definition.
(thesaurus)
'desire often extremely' is the wordplay.
'desire' becomes 'want' ('want' can be a synonym of 'desire'**).
'extremely' says to hollow out the word (remove centre letters).
'often' with its centre removed is 'on'.
'want'+'on'='WANTON'
(Other definitions for wanton that I've seen before include "Immoral; motiveless" , "Dissolute - gratuitously cruel" , "Irresponsible, lacking proper restraint" , "Dissolute and promiscuous" , "Thoughtlessly and unnecessarily cruel" .)