European energy needed to support southern ally (5)
I believe the answer is:
swede
'european' is the definition.
(Swede is an example)
'energy needed to support southern ally' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'needed to support' is a charade indicator (letters next to each other) (in a down clue, letters underneath support others).
'southern' becomes 'S' (abbreviation).
'ally' becomes 'wed' (both can mean to join).
's'+'wed'='swed'
'e' put after 'swed' is 'SWEDE'.
(Other definitions for swede that I've seen before include "Strindberg, for example" , "Foreigner" , "North European" , "Scandinavian national" , "Scandinavian - vegetable" .)