Informal alliance Alec entered into unwisely (7,8)
I believe the answer is:
entente cordiale
'informal alliance' is the definition.
'alec entered into unwisely' is the wordplay.
'unwisely' indicates anagramming the letters.
'alec'+'entered'+'into'='alecenteredinto'
'alecenteredinto' is an anagram of 'ENTENTE CORDIALE'.
(Other definitions for entente cordiale that I've seen before include "Agreement" , "Understanding" .)