Mistake to drop item from petition (3)
I believe the answer is:
ask
'petition' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ask that I've seen before include "Inquire" , "Issue invitations" , "Seek an answer" , "Invite (to a party)" , "Enquire" .)