Petition concerning expedition? (7)
I believe the answer is:
request
'petition' is the definition.
(I know that petition is a more specific form of the action request)
'concerning expedition?' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'expedition?' becomes 'quest' (I've seen this in other clues).
're'+'quest'='REQUEST'
(Other definitions for request that I've seen before include "Queer St (anag.)" , "Application" , "Suit" , "Plea" , "Call for" .)