Lass admits heart-throb is a tool (6)
I believe the answer is:
wrench
'tool' is the definition.
(wrench is a kind of tool)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for wrench that I've seen before include "Violent twist or pull" , "Violent twist - (adjustable) spanner" , "Sprain; spanner" , "Painful parting" , "Sort of spanner - twist it" .)