Demands nothing except energy, dedication and stamina at first (5)
I believe the answer is:
needs
'demands' is the definition.
(I know this)
'nothing except energy dedication and stamina at first' is the wordplay.
I cannot really understand how this works, but
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is present in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
The remaining letters 'eds' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for needs that I've seen before include "Wants, requires" , "Requires or lacks" , "shopping-list, say" , "'..... must when the devil drives'" , "Necessitates" .)