Energy shown by second Greek character turning up (8)
I believe the answer is:
momentum
'energy' is the definition.
(thesaurus)
'shown by second greek character turning up' is the wordplay.
'shown by' says to put letters next to each other.
'second' becomes 'moment' ('moment' can be a synonym of 'second'**).
'greek character' becomes 'mu' (letter from the Greek alphabet).
'turning up' is a reversal indicator.
'mu' back-to-front is 'um'.
'moment' next to 'um' is 'MOMENTUM'.
(Other definitions for momentum that I've seen before include "driving power" , "Impetus from motion" , "Impulse" , "Drive" , "Forward impetus" .)