Something put on a petition (11)
I believe the answer is:
application
'petition' is the definition.
(application is a kind of petition)
'something put on a' is the wordplay.
I cannot quite understand how this works, but
'on' could be 'at' (I've seen this in other clues) and 'at' is found in the answer.
'a' is within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for application that I've seen before include "Hard work" , "Request for job etc" , "industry?" , "computer package?" , "dressing!" .)