Greek character returning after short time offers driving force (8)
I believe the answer is:
Here is my best explanation:
'driving force' is the definition.
'greek character returning after short time' is the subsidiary indication.
'greek character' becomes 'mu'.
'returning' says the letters should be written in reverse.
'after' means one lot of letters go next to another.
'short time' becomes 'moment' (moment is a kind of short time**).
'mu' reversed gives 'um'.
'um' put after 'moment' is 'momentum'.
'offers' is the link.
(Other definitions for momentum that I've seen before include "Impelling force", "Impetus; driving force", "Impetus gained by movement, maybe downhill", "Impetus from motion", "Forward impetus" and "Driving force, impetus".)