Missionary replacing old plates (7)
I believe the answer is:
apostle
'missionary' is the definition.
(I've seen this before)
'replacing old plates' is the wordplay.
'replacing' indicates an anagram (placing again).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'o'+'plates'='oplates'
'oplates' anagrammed gives 'APOSTLE'.
(Other definitions for apostle that I've seen before include "Preacher of the gospel" , "E.g. St Paul" , "One of Christ's 12 original disciples" , "Good man" , "Burgess originally" .)