One male arbiter returned with a physicist (5)
I believe the answer is:
fermi
'physicist' is the definition.
(Enrico Fermi the nuclear physicist)
'one male arbiter returned' is the wordplay.
'one' becomes 'i' (Roman numeral).
'male' becomes 'm' (common abbreviation).
'arbiter' becomes 'ref' (I've seen this in another clue).
'returned' says the letters should be written in reverse.
'i'+'m'+'ref'='imref'
'imref' written backwards gives 'FERMI'.
'with a' acts as a link.
(Other definitions for fermi that I've seen before include "Enrico, Italian Nobel prize physicist" , "Enrico -----, Italian nuclear physicist (1901-1954); in the US from 1939" .)