Wood regularly found in vellum (3)
I believe the answer is:
elm
'wood' is the definition.
(elm is a type of tree)
'regularly found in vellum' is the wordplay.
'regularly found in' indicates alternate letters.
The alternate letters of 'vellum' are 'ELM'.
(Other definitions for elm that I've seen before include "wooden" , "Tree with a Dutch disease" , "One [TYPE OF TREE]" , "Strange tree for Mel" , "Tall shade tree" .)