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. Maybe there's an association between them I don't understand?
'using stone for weight with heft' is the wordplay.
I cannot really understand how this works, but
'using' could be 'on' (someone using drugs is 'on' them) and 'on' is found in the answer.
'stone' could be 'st' (abbreviation) and 'st' is present in 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" .)