Twopence piece is visible (4)
I believe the answer is:
open
'visible' is the definition.
('open' can be similar in meaning to 'visible')
'twopence piece' is the wordplay.
'piece' says the answer is hidden in the clue.
'OPEN' is hidden in the letters of 'twopence'.
'is' is the link.
(Other definitions for open that I've seen before include "Without concealment" , "Unrestricted; ready for business" , "Undecided" , "simple" , "begin to > crack" .)