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 wordplay.
'greek character' becomes 'mu' (letter from the Greek alphabet).
'returning' shows that the letters should be reversed in order.
'after' is a charade indicator (letters next to each other).
'short time' becomes 'moment' (moment is a kind of short time**).
'mu' written backwards gives 'um'.
'um' put after 'moment' is 'MOMENTUM'.
'offers' is the link.
(Other definitions for momentum that I've seen before include "driving power", "Impelling force", "Corbyn supporters", "Impetus gained by movement, maybe downhill", "Impetus from motion", "Forward impetus", "Corbynites", "Impetus; driving force", "Impetus gained by a moving object" and "Drive".)