Brisk energy (3)
I believe the answer is:
pep
'brisk energy' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pep that I've seen before include "Palindrome for vigour" , "Get-up-and-go" , "Energy, liveliness" , "Palindromic vim and vigour" , "Vim and vitality" .)