Imagine being armed, possibly (5)
I believe the answer is:
dream
'imagine' is the definition.
(dreaming is a kind of imagining)
'armed possibly' is the wordplay.
'possibly' indicates an anagram.
'armed' is an anagram of 'DREAM'.
'being' acts as a link.
(Other definitions for dream that I've seen before include "Reverie or nightmare" , "person to good to be true" , "Sleeping thoughts" , "Vision, fantasy" , "Nocturnal vision" .)