Uncle Arthur is shortly vague (7)
I believe the answer is:
unclear
'vague' is the definition.
(similar in meaning)
'uncle arthur is shortly' is the wordplay.
'arthur' becomes 'art' (I've seen this in other clues).
'is shortly' means to remove the last letter.
'art' with its final letter taken off is 'ar'.
'uncle'+'ar'='UNCLEAR'
(Other definitions for unclear that I've seen before include "Imprecise, hazy" , "Not obvious, obscure" , "Indistinct" , "An ulcer (anag)" , "Not apparent" .)