In return a number get nothing (3)
I believe the answer is:
nil
'nothing' is the definition.
(eg nil is zero in football)
'in return a number' is the wordplay.
'return' shows that the letters should be reversed in order.
'a number' becomes 'l' (fifty in Roman numerals).
'in' back-to-front is 'ni'.
'ni'+'l'='NIL'
'get' is the link.
(Other definitions for nil that I've seen before include "0" , "Zero, zilch" , "duck" , "Non-existent" , "Love" .)