Sweetener — because some in the morning needing energy (9)
I believe the answer is:
aspartame
'sweetener' is the definition.
(aspartame is a kind of sweetener)
'because some in the morning needing energy' is the wordplay.
'because' becomes 'as' (I've seen this in other clues).
'some' becomes 'part' (**).
'in the morning' becomes 'am'.
'needing' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mca).
'as'+'part'+'am'+'e'='ASPARTAME'
(Other definitions for aspartame that I've seen before include "Strong artificial sweetener -- same apart (anag)" , "What sweetens" , "Sugar substitute" .)