The first right amount of weight is first detected and fully defeated (8)
I believe the answer is:
trounced
'defeated' is the definition.
('trounce' can be a synonym of 'defeat')
'the first right amount of weight is first detected and fully' is the wordplay.
I cannot quite see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
'right' could be 'r' (common abbreviation) and 'r' is found within the answer.
'amount' could be 'ounce' (ounce is a kind of amount) and 'ounce' is found within the answer.
'first detected' could be 'd' (1st letter of 'detected') and 'd' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for trounced that I've seen before include "Defeated decisively" , "Not cured after receiving heavy beating" , "Beat hollow" , "Thrashed, beat heavily" .)