Rating part of Mass, calls back to cover front of altar (8,6)

'rating' is the definition.
('rating' can mean a non-commissioned navy sailor)

'part of mass calls back to cover front of altar' is the subsidiary indication.
'part of mass' becomes 'ordinary' (component of Catholic mass).
'calls' becomes 'names' ('name' can be a synonym of 'call'**).
'back' says the letters should be written backwards.
'to cover' is an insertion indicator.
'front of' indicates taking the first letters.
The first letter of 'altar' is 'a'.
'names' backwards is 'seman'.
'seman' going around 'a' is 'seaman'.
'ordinary'+'seaman'='ordinaryseaman'