Souvenir pieces impounded by London police ring (7)
I believe the answer is:
memento
'souvenir' is the definition.
('memento' can be a synonym of 'souvenir')
'pieces impounded by london police ring' is the wordplay.
'pieces' becomes 'men' ('man' can be a synonym of 'piece').
'impounded by' is an insertion indicator.
'london police' becomes 'met' (London Metropolitan Police).
'ring' becomes 'o'.
'men' inserted into 'met' is 'mement'.
'mement'+'o'='MEMENTO'
(Other definitions for memento that I've seen before include "trophy" , "This reminds" , "Reminder that death is inevitable" , "Souvenir, reminder of past event" , "Something kept" .)