Weight, energy and speed unaltered (8)
I believe the answer is:
emphasis
'weight' is the definition.
(thesaurus)
'energy and speed unaltered' 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).
'unaltered' becomes 'asis' (I've seen this in other clues).
'e'+'mph'+'asis'='EMPHASIS'
(Other definitions for emphasis that I've seen before include "Stress given to a word" , "Intensity of expression" , "Stress, accentuation" , "Accentutation" , "Special importance or prominence given to something" .)