Black stuff regularly found in tea urn (3)
I believe the answer is:
tar
'black stuff' is the definition.
(tar is a black substance)
'regularly found in tea urn' is the wordplay.
'regularly found in' indicates alternate letters.
'tea'+'urn'='teaurn'
The alternating letters of 'teaurn' are 'TAR'.
(Other definitions for tar that I've seen before include "Viscous liquid; sailor" , "Informal sailor or road surfacing material" , "Tobacco residue; sailor" , "A sailor; a preservative" , "Dark flammable liquid" .)