Card-game authority (5)
I believe the answer is:
hoyle
'card-game authority' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hoyle that I've seen before include "English author, expert on card games, d.1769 - English astrophysicist, d. 2001" , "Sir Fred(erick) -, British astronomer" , "English astronomer who rejected the 'Big Bang' theory of the origins of the universe, d. 2001" , "English astrophysicist, who coined the term "big bang", d. 2001" , "cosmologist" .)