Cardinal uses ancient entry occasionally (6)
I believe the answer is:
ninety
'cardinal' is the definition.
(similar in meaning)
'ancient entry occasionally' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'ancient'+'entry'='anciententry'
The alternate letters of 'anciententry' are 'NINETY'.
'uses' acts as a link.
(Other definitions for ninety that I've seen before include "XC (in ancient Rome)" , "Three thirties" , "Cardinal" , "XC in Roman numerals" , "Figure in inn, yet, strangely" .)