Seal petition in bag (6)
I believe the answer is:
gasket
'seal' is the definition.
(gasket is a kind of seal)
'petition in bag' is the wordplay.
'petition' becomes 'ask'.
'in' is an insertion indicator.
'bag' becomes 'get' (I am not sure about this - if you are sure you should believe this answer much more).
'ask' put into 'get' is 'GASKET'.
Can you help me to learn more?
(Other definitions for gasket that I've seen before include "Seal (in, eg, engine)" , "engineering part" , "Sealing sheet in an engine" , "Flat sheet used as engine seal" , "Seal between metal faces" .)