Papers regularly restricting long letters (7)
I believe the answer is:
aitches
'letters' is the definition.
(aitch is a kind of letter)
'papers regularly restricting long' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'restricting' means one lot of letters goes inside another.
'long' becomes 'itch' (to long or itch for something is to desire it).
The alternate letters of 'papers' are 'aes'.
'aes' placed around 'itch' is 'AITCHES'.
(Other definitions for aitches that I've seen before include "Two of which are in Hythe" , "Letters" , "They are regularly dropped in the East End" .)