Optimism lacks energy to leap (3)
I believe the answer is:
hop
'leap' is the definition.
(hopping is a kind of leaping)
'optimism lacks energy' is the wordplay.
'optimism' becomes 'hope' (I've seen this before).
'lacks' indicates named letters should be taken away.
'energy' becomes 'e' (physics symbol as in E=mc2).
'hope' with 'e' removed is 'HOP'.
'to' is the link.
(Other definitions for hop that I've seen before include "Climbing plant used to flavour beer (3)" , "Skip; beer flavouring ingredient" , "Dancing party (informal)" , "Jump on a pogo stick" , "Climbing plant - jump!" .)