Guard's content to have petition (7)
I believe the answer is:
entreat
'petition' is the definition.
(I've seen this before)
'guard's content to have' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'content' says to take the centre.
'to have' becomes 'eat' (eating is a kind of having).
The middle letters of 'sentry' are 'entr'.
'entr'+'eat'='ENTREAT'
(Other definitions for entreat that I've seen before include "Beg earnestly, implore" , "Pray" , "Will's pass" , "Request earnestly" , "Beg earnestly, beseech" .)