Declare in favour of petition (8)
I believe the answer is:
proclaim
'declare' is the definition.
(I know this)
'in favour of petition' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'petition' becomes 'claim' (claim is a kind of petition).
'pro'+'claim'='PROCLAIM'
(Other definitions for proclaim that I've seen before include "Herald" , "Speak out" , "Announce officially or declare publicly" , "Make a public announcement" , "Reveal" .)