Energy, informally
I believe the answer is:
oomph
'energy informally' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oomph that I've seen before include "Get-up-and-go" , "Go" , "Informal energy or enthusiasm, usually in a girl" , "Enthusiasm - vigour - sex appeal" , "sex-appeal" .)