Number playing around in nets, cutting 50 per cent (4)
I believe the answer is:
nine
'number' is the definition.
(nine is a number)
'playing around in nets cutting 50 per cent' is the wordplay.
'playing around' indicates an anagram.
'cutting 50 per cent' suggests halving the letters.
'nets' halved is 'ne'.
'in' with letters rearranged gives 'ni'.
'ni'+'ne'='NINE'
(Other definitions for nine that I've seen before include "3x3" , "Number IX" , "Number of lepers who failed to return" , "Number of Muses and days in a Novena" , "Square next up after four" .)