What has London banks acting badly during trial, mostly (6)
I believe the answer is:
thames
'what has london banks' is the definition.
(river with banks in London)
'acting badly during trial mostly' is the wordplay.
'acting badly' becomes 'ham' (to ham is to overact).
'during' indicates putting letters inside.
'trial' becomes 'test' (synonyms).
'mostly' means to remove the last letter.
'test' with its final letter removed is 'tes'.
'ham' going inside 'tes' is 'THAMES'.
(Other definitions for thames that I've seen before include "London's river (6)" , "London river" , "River at Henley" , "River at Lechlade" , "Sam the river" .)