Catch a number up (3)
I believe the answer is:
net
'catch' is the definition.
(netting is a kind of catching)
'a number up' is the wordplay.
'a number' becomes 'ten' (ten is a kind of number).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'ten' written backwards gives 'NET'.
(Other definitions for net that I've seen before include "After taxes" , "Fish trap and type of goal" , "Trap; open-weave fabric" , "Snare or trap" , "Tennis court feature" .)