Forge with great speed sealing iron chest, initially (9)
I believe the answer is:
fabricate
'forge' is the definition.
(I've seen this before)
'great speed sealing iron chest initially' is the wordplay.
'great' becomes 'fab' (short for fabulous).
'speed' becomes 'rate' (the speed at which something happens).
'sealing' means one lot of letters goes inside another (inserted letters are sealed inside).
'iron chest initially' becomes 'ic' (I can't explain this - if you can you should give a lot more credence to this answer).
'rate' going around 'ic' is 'ricate'.
'fab'+'ricate'='FABRICATE'
'with' is the link.
Can you help me to learn more?
(Other definitions for fabricate that I've seen before include "forge" , "Make up; manufacture" , "Tell lies" , "Invent or manufacture a story" , "Construct or invent" .)