One’s completely tin-plated coffer (7)
I believe the answer is:
'coffer' is the definition.
(I know that coffer can be written as caisson)
'one's completely tin-plated' is the wordplay.
'one's' becomes 'I's' (Roman numeral).
'completely' becomes 'so' (both can mean 'very much').
'tin' becomes 'can' (synonyms).
'plated' means one lot of letters goes inside another (plated can mean covered or coated).
'isso' inserted into 'can' is 'CAISSON'.
(Other definitions for caisson that I've seen before include "So I scan the ammunition wagon" , "Structure excluding water from construction" , "watertight chamber" , "Water-excluding case" .)