Foreign Legion can be agreeable (9)
I believe the answer is:
congenial
'agreeable' is the definition.
(synonyms)
'foreign legion can' is the wordplay.
'foreign' is an anagram indicator (foreign order of the letters).
'legion'+'can'='legioncan'
'legioncan' anagrammed gives 'CONGENIAL'.
'be' is the link.
(Other definitions for congenial that I've seen before include "Pleasant and agreeable" , "Harmonious and friendly" , "relaxing" , "Having similar interests" , "Suited to one's taste" .)