They're authorised to certify stuff: not something to do with astrology (8)
I believe the answer is:
notaries
'they're authorised to certify stuff' is the definition.
'not something to do with astrology' is the wordplay.
'something to do with astrology' becomes 'aries' (sign of the zodiac).
'not'+'aries'='NOTARIES'
(Other definitions for notaries that I've seen before include "Legal experts" , "Offices who draw up and witness documents" , "They witness official documents" , "Legal clerks" , "Public officials" .)