Eliminate periods with little energy (5)
I believe the answer is:
erase
'eliminate' is the definition.
(I've seen this before)
'periods with little energy' is the wordplay.
'periods' becomes 'eras' (periods of time).
'with' says to put letters next to each other.
'little' indicates taking the first letters.
The initial letter of 'energy' is 'e'.
'eras'+'e'='ERASE'
(Other definitions for erase that I've seen before include "Put out of mind" , "Delete" , "Remove evidence of" , "Take out" , "To remove all trace" .)