Occasionally Doug breeds monsters (5)
I believe the answer is:
ogres
'monsters' is the definition.
('ogre' can be a synonym of 'monster')
'occasionally doug breeds' is the wordplay.
'occasionally' means one should take alternating letters.
'doug'+'breeds'='dougbreeds'
The alternate letters of 'dougbreeds' are 'OGRES'.
(Other definitions for ogres that I've seen before include "Fairytale giants" , "Gorse (anag.)" , "intimidating people" , "Beasts" , "Giants in fairytales" .)