Fun with a heavy weight (7)
I believe the answer is:
ballast
'weight' is the definition.
(I know that Ballast is a type of ballast)
'fun with a heavy' is the wordplay.
'fun' becomes 'blast'.
'with' means one lot of letters goes inside another.
'a heavy' becomes 'al' (I can't explain this - if you can you should give a lot more credence to this answer).
'blast' enclosing 'al' is 'BALLAST'.
Can you help me to learn more?
(Other definitions for ballast that I've seen before include "Heavy material for steadying ship" , "Material for railway bed" , "Heavy stuff used to stabilise boat" , "Heavy material taken aboard to steady ship" , "Heavy material used to stabilise a ship" .)