Pure speed of light expedition (6)
I believe the answer is:
chaste
'pure' is the definition.
(chaste means abstaining from sex)
'speed of light expedition' is the wordplay.
'speed of light' becomes 'c' (as in E=mc2).
'expedition' becomes 'haste' (expedition is expediting, doing something quickly).
'c'+'haste'='CHASTE'
(Other definitions for chaste that I've seen before include "Modest and decent" , "Sexually virtuous" , "Pure - virtuous" , "'Celibate, virginal (6)'" , "Decorous" .)