Cash card number son secures (4)
I believe the answer is:
pins
'secures' is the definition.
(as in pinning something down)
'cash card number son' is the wordplay.
'cash card number' becomes 'PIN' (as in a PIN for a bank card).
'son' becomes 's' (genealogical abbreviation for son).
'pin'+'s'='PINS'
(Other definitions for pins that I've seen before include "security codes" , "Stocking fillers" , "With needles, they create a tingling sensation" , "Wire fasteners" .)