Heavy weight not a stone, on reflection (7)
I believe the answer is:
megaton
'heavy weight' is the definition.
'megaton' can be an answer for 'weight' (I've seen this before). I'm unsure of the 'heavy' bit.
'not a stone on reflection' is the wordplay.
'stone' becomes 'gem' (gemstone).
'on reflection' says the letters should be written backwards.
'not'+'a'+'gem'='notagem'
'notagem' backwards is 'MEGATON'.
Can you help me to learn more?
(Other definitions for megaton that I've seen before include "Measure of explosive power - montage (anag)" , "Explosive power equivalent to one million tons of TNT" , "Powerful explosive unit" , "Unit of explosive power" , "it weighs a lot" .)