It's inside cover, though not hidden (5)
I believe the answer is:
overt
'not hidden' is the definition.
(I've seen this before)
'it's inside cover though' is the wordplay.
'it's inside' indicates the answer is hidden within the clue.
'OVERT' is hidden within 'cover though'.
(Other definitions for overt that I've seen before include "Explicit" , "Open,up front" , "Done in an obvious way" , "Open and candid" , "Open, not 1 across" .)