Gift contains first radioactive element (5)
I believe the answer is:
boron
'element' is the definition.
(boron is a kind of element)
'gift contains first radioactive' is the wordplay.
'gift' becomes 'boon' (I've seen this before).
'contains' means one lot of letters goes inside another.
'first' suggests taking the first letters.
The first letter of 'radioactive' is 'r'.
'boon' placed around 'r' is 'BORON'.
(Other definitions for boron that I've seen before include "component of steel" , "Chemical element, symbol B" , "Atomic no 5" , "element needed for steel" , "Metalloid element, B" .)