The enclosure of a threepenny bit (3)
I believe the answer is:
pen
'the enclosure' is the definition.
(pen is a kind of enclosure)
'of a threepenny bit' is the wordplay.
'of' indicates a hidden word.
'PEN' can be found hidden inside 'threepenny bit'.
(Other definitions for pen that I've seen before include "American jug?" , "Animal enclosure" , "Prison" , "'The ... is mightier than the sword'" , "International Association of poets, playwrights etc" .)