A number turning up in trap (3)
I believe the answer is:
net
'trap' is the definition.
(net is a type of trap)
'a number turning up' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'turning up' shows that the letters should be reversed in order.
'ten' backwards is 'NET'.
'in' acts as a link.
(Other definitions for net that I've seen before include "Fish trap and type of goal" , "catch fish" , "Open-meshed fabric" , "Fine open-weave fabric" , "Lattice" .)