Catch turns up a whole number (3)
I believe the answer is:
ten
'a whole number' is the definition.
(ten is a kind of whole number)
'catch turns up' is the wordplay.
'catch' becomes 'net' (netting is a kind of catching).
'turns up' shows that the letters should be reversed in order.
'net' written backwards gives 'TEN'.
(Other definitions for ten that I've seen before include "Half a score" , "Very young" , "X-WORD" , "No of sides of a decagon" , "Base of decimal system" .)