Destructive type left at rear of London museum (6)
I believe the answer is:
vandal
'destructive type' is the definition.
(someone who destroys or damages property)
'left at rear of london museum' is the wordplay.
'left' becomes 'L' (common abbreviation).
'at rear of' means one lot of letters go next to another (some letters go behind others).
'london museum' becomes 'Vanda' (the Victoria and Albert Museum).
'l' after 'vanda' is 'VANDAL'.
(Other definitions for vandal that I've seen before include "Old German" , "Barbarian - wrecker" , "destructive member of the public" , "Deliberate destroyer" , "Defacer or wrecker" .)