The Ma's over in London (6)
I believe the answer is:
thames
'the ma's over in london' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for thames that I've seen before include "Longest river in England" , "River at Goring" , "London's river (6)" , "River of southern England" , "It flows through London" .)