Stress online speed's unaltered (8)
I believe the answer is:
emphasis
'stress' is the definition.
(I know that stress can be written as emphasis)
'online speed's unaltered' is the wordplay.
'online' becomes 'e' (as in e-mail or e-commerce).
'speed' becomes 'mph' (miles per hour).
'unaltered' becomes 'as is' (ie something left 'as is').
'e'+'mph'+'asis'='EMPHASIS'
(Other definitions for emphasis that I've seen before include "Importance; priority" , "Special importance or significance" , "Stress, special importance" , "Stress given to a word" , "Intensity of expression" .)