Be in favour of energy worker coming early (10)
I believe the answer is:
beforehand
'coming early' is the definition.
'be in favour of energy worker' is the wordplay.
'in favour of' becomes 'for' (I've seen this before).
'energy' becomes 'e' (physics symbol as in E=mc2).
'worker' becomes 'hand' (hand is a kind of worker).
'be'+'for'+'e'+'hand'='BEFOREHAND'
(Other definitions for beforehand that I've seen before include "In advance" , "Previously" , "Earlier" .)