Take a number back (3)
I believe the answer is:
net
'take' is the definition.
(thesaurus)
'a number back' is the wordplay.
'a number' becomes 'ten' (ten is a kind of number).
'back' says the letters should be written in reverse.
'ten' backwards is 'NET'.
(Other definitions for net that I've seen before include "It turns in every direction" , "What separates court opponents" , "device to divide the court" , "After taxes" , "Mesh; entrap" .)