Bits regularly itch inside plaster cast (9)
I believe the answer is:
particles
'bits' is the definition.
'regularly itch inside plaster cast' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'inside' indicates putting letters inside.
'cast' indicates an anagram (letters are dispersed or cast around).
The alternate letters of 'itch' are 'ic'.
'plaster' is an anagram of 'partles'.
'ic' placed within 'partles' is 'PARTICLES'.
(Other definitions for particles that I've seen before include "Minute pieces of matter" , "Tiny pieces" , "Scraps" .)