Understanding ten/teen difference (7)
I believe the answer is:
entente
'understanding' is the definition.
(an entente is an alliance or understanding)
'tenteen difference' is the wordplay.
'difference' indicates anagramming the letters.
'tenteen' anagrammed gives 'ENTENTE'.
(Other definitions for entente that I've seen before include "A friendly understanding between countries, may be cordial" , "International agreement" , "Political agreement" , "Friendly agreement between countries" , "friendly relations" .)