Fur regularly dear, a source of wealth (6)
I believe the answer is:
ermine
'fur' is the definition.
(I know that ermine is a type of fur)
'regularly dear a source of wealth' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'a source of wealth' becomes 'mine' (I've seen this before).
The alternate letters of 'dear' are 'er'.
'er'+'mine'='ERMINE'
(Other definitions for ermine that I've seen before include "special trimmings" , "In mere strange fur" , "White fur from kind of 25 Across - judges may wear it" , "garb for nobility" , "Fur-bearing mammal" .)