Was a killer from Isleworth? (4)
I believe the answer is:
slew
'was a killer' is the definition.
(past tense of 'slay')
'from isleworth?' is the wordplay.
'from' indicates the answer is hidden within the clue.
'SLEW' can be found hidden inside 'isleworth'.
(Other definitions for slew that I've seen before include "making sharp turn" , "Twist sideways" , "Butchered" , "greatly impressed" , "a large number in America" .)