The atmosphere is first class right before the Church service (3,4)

'the atmosphere' is the definition.
(air mass is a kind of atmosphere)

'first class right before the church service' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'right' becomes 'r' (common abbreviation).
'before' says to put letters next to each other.
'the church service' becomes 'mass' (Mass is a type of church service).
'ai'+'r'+'mass'='AIR MASS'

'is' acts as a link.