Energy conservationists doubled European understanding (7)
I believe the answer is:
entente
'understanding' is the definition.
(an entente is an alliance or understanding)
'energy conservationists doubled european' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'conservationists' becomes 'NT' (abbreviation for National Trust).
'doubled' indicates the doubling of some letters.
'european' becomes 'e' (abbreviation e.g. EU).
'e'+'nt'='ent'
'ent' doubled is 'entent'.
'entent'+'e'='ENTENTE'
(Other definitions for entente that I've seen before include "Teen ten involved in friendly understanding between countries" , "Informal alliance" , "International agreement" , "that brings nations closer together" , "Diplomatic agreement" .)