Promising girl full of dynamic energy (6)
I believe the answer is:
likely
'promising' is the definition.
(similar in meaning)
'girl full of dynamic energy' is the wordplay.
'girl' becomes 'Lily' (girl's name).
'full of' means one lot of letters goes inside another.
'dynamic energy' becomes 'KE' (abbreviation for kinetic energy).
'lily' enclosing 'ke' is 'LIKELY'.
(Other definitions for likely that I've seen before include "In all probability" , "Ilkley (anag.)" , "In prospect" , "Quite possible" , "Has a good chance of being the case" .)