Writer of an enclosure for somebody (6)
I believe the answer is:
'writer' is the definition.
(penman is a kind of writer)
'an enclosure for somebody' is the wordplay.
'an enclosure' becomes 'pen' (pen is a kind of enclosure).
'for' says to put letters next to each other.
'somebody' becomes 'man'.
'of' is the link.
(Other definitions for penman that I've seen before include "author" , "16 dn" .)