That's the stuff, man! (5)
I believe the answer is:
serge
This clue is a double definition.
'that's the stuff' is the first definition.
(I've seen this before)
'man' is the second definition.
(Other definitions for serge that I've seen before include "Twilled fabric; name" , "Stuff" , "Durable fabric" , "Twilled woollen fabric" , "A twill fabric" .)