Fast living (5)
I believe the answer is:
quick
I believe this clue is a double definition.
'fast' is the first definition.
('quick' can be similar in meaning to 'fast')
'living' is the second definition.
(thesaurus)
(Other definitions for quick that I've seen before include "Swift" , "Part of fingernail" , "and 9: This is it!" , "Prompt; intelligent" , "living" .)