Uranium, nickel and tin good, becoming one (7)
I believe the answer is:
uniting
'becoming one' is the definition.
(uniting is a kind of becoming one)
'uranium nickel and tin good' is the wordplay.
'uranium' becomes 'u' (U is the chemical symbol for uranium).
'nickel' becomes 'ni' (Ni is the chemical symbol for nickel).
'and' means one lot of letters go next to another.
'good' becomes 'g' (abbreviation).
'u'+'ni'+'tin'+'g'='UNITING'
(Other definitions for uniting that I've seen before include "Combining into one" , "becoming one" , "Coming together" , "wedding?" , "Victor --, French author" .)