Polly run over, which stops involvement (6)
I believe the answer is:
parrot
'polly' is the definition.
(thesaurus)
'run over which stops involvement' is the wordplay.
'run over' shows that the letters should be reversed in order (I've seen 'running over' mean this (the letters turn over to the opposite direction)).
'which' becomes 'or' (I can't justify this - if you can you should give a lot more credence to this answer).
'stops' means one lot of letters goes inside another.
'involvement' becomes 'part' (part is a kind of involvement).
'or' reversed gives 'ro'.
'ro' going inside 'part' is 'PARROT'.
Can you help me to learn more?
(Other definitions for parrot that I've seen before include "Repeat unintelligently" , "Brightly coloured tropical bird" , "A flighty creature" , "Brightly-coloured tropical bird" , "Long John Silver's Captain Flint" .)