Second prime (5)
I believe the answer is:
three
'second prime' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for three that I've seen before include "Start to count back, briefly" , "Number of times N appears in this completed grid" , "Snooker score for potting green ball" , "... number of his "Steps to Heaven"" , "Number of strikes before you're out" .)