Lefties' second letter to Greeks returned (8)
I believe the answer is:
momentum
'lefties' second letter to greeks returned' is the definition.
I can't tell whether this defines the answer.
'second letter to greeks returned' is the wordplay.
'second' becomes 'moment' (synonyms).
'letter to greeks' becomes 'mu' (letter from the Greek alphabet).
'returned' shows that the letters should be reversed in order.
'mu' backwards is 'um'.
'moment'+'um'='MOMENTUM'
Can you help me to learn more?
(Other definitions for momentum that I've seen before include "Impetus gained by a moving object" , "Impelling force" , "Corbyn supporters" , "Impetus from motion" , "Corbynites" .)