Smashing individual, say, in line behind museum (6)
I believe the answer is:
vandal
'smashing individual say' is the definition.
(a vandal might smash things)
'line behind museum' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'behind' says to put letters next to each other.
'museum' becomes 'V and A' (Victoria and Albert Museum).
'l' after 'vanda' is 'VANDAL'.
'in' is the link.
(Other definitions for vandal that I've seen before include "Person causing destruction to property" , "One who causes wilful damage to property" , "Barbarian - wrecker" , "Person causing wanton damage" , "Destructive type" .)