Fourth prime (5)
I believe the answer is:
seven
'fourth prime' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for seven that I've seen before include "Film" , "Wonders of the World - Deadly Sins" , "Numeral" , "So many ages of man (AYLI)" , "Number of sisters in Sussex" .)