Frank is hiding in Copenhagen! (4)
I believe the answer is:
open
'frank' is the definition.
(synonyms)
'hiding in copenhagen' is the wordplay.
'hiding' indicates the answer is hidden within the clue.
'OPEN' is hidden in the letters of 'in copenhagen'.
'is' acts as a link.
(Other definitions for open that I've seen before include "Plain" , "such a verdict?" , "simple" , "Accessible - candid" , "Not closed or obstructed" .)