Leg part regularly brown in beam (8)
I believe the answer is:
shinbone
'leg part' is the definition.
('shinbone' is part of 'leg')
'regularly brown in beam' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'in' is an insertion indicator.
'beam' becomes 'shine' (synonyms).
The alternate letters of 'brown' are 'bon'.
'bon' inserted into 'shine' is 'SHINBONE'.
(Other definitions for shinbone that I've seen before include "It is also called the tibia" , "bit of leg" , "The less formal tibia" , "that supports member" .)