Leading statistician still finding a prime number (5)
I believe the answer is:
seven
'a prime number' is the definition.
(I've seen this in another clue)
'leading statistician still' is the wordplay.
'leading' suggests taking the first letters (the leading letter of the word).
'still' becomes 'even' (synonyms).
The initial letter of 'statistician' is 's'.
's'+'even'='SEVEN'
'finding' is the link.
(Other definitions for seven that I've seen before include "Number of seas sailed?" , "Number of sisters in Sussex" , "Cardinal" , "Wonders of the World - Deadly Sins" , "Number of sins, seas or dwarves" .)