Chest carefully opened by volunteer (6)
I believe the answer is:
coffer
'chest' is the definition.
(I know that coffer is a type of chest)
'carefully opened by volunteer' is the wordplay.
'carefully opened' becomes 'c' (I can't justify this - if you can you should give a lot more credence to this answer).
'by' means one lot of letters go next to another.
'volunteer' becomes 'offer' (synonyms).
'c'+'offer'='COFFER'
Can you help me to learn more?
(Other definitions for coffer that I've seen before include "jewellery store?" , "A large strong box for valuables" , "Treasure chest" , "safe" , "Money chest sounds like one with lung trouble" .)