Two soldiers united in musical (4)
I believe the answer is:
gigi
'musical' is the definition.
(1973 musical)
'two soldiers united' is the wordplay.
'two soldiers' means two replacements for 'soldier'.
'soldier' becomes 'GI' (GI can mean a US soldier).
'soldier' becomes 'gi' (GI can mean a US soldier).
'united' is a charade indicator (letters next to each other).
'gi'+'gi'='GIGI'
'in' is the link.
(Other definitions for gigi that I've seen before include "1958 film, winner of 9 Academy Awards" , "Musical, winner of nine Academy Awards" , "musical from the 1950s" , "Lerner/Loewe musical" , "Movie" .)