Forceful energy and speed at one hundred (8)
I believe the answer is:
emphatic
'forceful' is the definition.
('emphatic' can be similar in meaning to 'forceful')
'energy and speed at one hundred' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'and' means one lot of letters go next to another.
'speed' becomes 'mph' (miles per hour).
'one' becomes 'i' (Roman numeral).
'hundred' becomes 'c' (Roman numeral).
'e'+'mph'+'at'+'i'+'c'='EMPHATIC'
(Other definitions for emphatic that I've seen before include "Able to experience another's feelings" , "speaking firmly" , "In no uncertain terms" , "Forceful - pronounced" , "being stressed" .)