Crossword Genius

Peter, Christ’s first volunteer (6)

Ross

I believe the answer is:

coffer

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'peter' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'christ's first volunteer' is the wordplay.
'christ's first' becomes 'c' (1st letter of 'christ').
'volunteer' becomes 'offer' (synonyms).
'c'+'offer'='COFFER'

Can you help me to learn more?

(Other definitions for coffer that I've seen before include "Small chest for keeping valuables" , "A large strong box for valuables" , "Strong box at the treasury" , "Strongbox" , "Money chest sounds like one with lung trouble" .)

I've seen this clue in The Sunday Times.
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