Lester is running about, as always full of energy (8)
I believe the answer is:
'full of energy' is the definition.
(I've seen this before)
'lester is running about' is the wordplay.
'running about' indicates anagramming the letters.
'lesteris' is an anagram of 'TIRELESS'.
'as always' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tireless that I've seen before include "Never wearying" , "Full of energy" , "Having inexhaustible energy" , "Indefatigable, unflagging" , "Permanently lively" .)