As one new service in heart of Greece (2,5)
I believe the answer is:
en masse
'as one' is the definition.
(both can mean as a group)
'new service in heart of greece' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'service' becomes 'mass' (I've seen this in other clues).
'in' indicates putting letters inside.
'heart of' means to look at the middle letters.
The middle of 'greece' is 'ee'.
'n'+'mass'='nmass'
'nmass' inserted into 'ee' is 'EN MASSE'.
(Other definitions for en masse that I've seen before include "in concert" , "Sees man (anag.)" , "As a body (Fr.)" , "In force" , "In a group U altogether" .)