It's my cunning, being vague (5)
I believe the answer is:
misty
'vague' is the definition.
(thesaurus)
'it's my cunning' is the wordplay.
'cunning' indicates an anagram.
'its'+'my'='itsmy'
'itsmy' is an anagram of 'MISTY'.
'being' is the link.
(Other definitions for misty that I've seen before include "unclear" , "Cloudy, foggy" , "Indistinct in outline" , "Foggy; dim in outline" , "Slightly fogged" .)