Lassie clutches painter, shedding a tear (6)
I believe the answer is:
wrench
'a tear' is the definition.
(wrench can mean to tear or pull)
'lassie clutches painter shedding' is the wordplay.
'lassie' becomes 'wench' (both can informally mean a young woman).
'clutches' is an insertion indicator.
'painter shedding' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'wench' going around 'r' is 'WRENCH'.
(Other definitions for wrench that I've seen before include "Jerky pulling movement" , "Violent twist or pull" , "Kind of spanner or severe twist" , "Sprain; spanner" , "Sort of spanner - twist it" .)