Four Swedes in prison sharing a vegetable (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

ABBA (=four Swedes, i.e. the four singers/musicians that make up the group) in CAGE (=prison); there are two x “a” in “cabbage”, not three x “a”, so one x “a” has to be shared when “ABBA” is placed in “CAGE”