Not working with number (3)
I believe the answer is:
two
'number' is the definition.
(two is a kind of number)
'not working with' is the wordplay.
'with' indicates a hidden word.
'TWO' can be found hidden inside 'not working'.
(Other definitions for two that I've seen before include "A low number" , "Number in pair" , "Item" , "Fifth root of thirty-two" , "Pair, deuce" .)