Mistakenly using stone for weight with heft (8)
I believe the answer is:
strongly
'mistakenly' is the definition.
Both the definition and answer are adverbs. Perhaps you can see an association between them that I don't see?
'using stone for weight with heft' is the wordplay.
I cannot quite understand how this works, but
'using' could be 'on' (someone using drugs is 'on' them) and 'on' is present in the answer.
'stone' could be 'st' (abbreviation) and 'st' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for strongly that I've seen before include "Firmly or with great power" , "With force" , "In a powerful manner" , "In a forceful manner" , "hard" .)