Trap to catch a number following (4)
I believe the answer is:
next
'following' is the definition.
(I know that following can be written as next)
'trap to catch a number' is the wordplay.
'trap' becomes 'net' (net is a kind of trap).
'to catch' means one lot of letters goes inside another.
'a number' becomes 'x' (X is an example).
'net' enclosing 'x' is 'NEXT'.
(Other definitions for next that I've seen before include "The one following or adjacent" , "Closest" , "Coming nearest in time or sequence" , "First in line" , "Coming immediately" .)