Frolic with energy in springtime -- that's nonsense! (8)
I believe the answer is:
malarkey
'nonsense' is the definition.
(I've seen this before)
'frolic with energy in springtime' is the wordplay.
'frolic' becomes 'lark' ('lark' can be a synonym of 'frolic'**).
'with' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mca).
'in' is an insertion indicator.
'springtime' becomes 'may'.
'lark'+'e'='larke'
'larke' put within 'may' is 'MALARKEY'.
'that's' acts as a link.
(Other definitions for malarkey that I've seen before include "Absurd talk" , "Slangy humbug or nonsense - sounds like Irish name" , "Idle words" , "Cock" , "Nonsensical stuff" .)