Catch a number climbing (3)
I believe the answer is:
net
'catch' is the definition.
(net can mean to catch)
'a number climbing' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'ten' in reverse letter order is 'NET'.
(Other definitions for net that I've seen before include "Open weave fabric" , "catch fish" , "Amount remaining after deductions" , "Capture" , "Tennis equipment" .)