Informed lions about foreign character (7)
I believe the answer is:
upsilon
'foreign character' is the definition.
(Greek letter)
'informed lions about' is the wordplay.
'informed' becomes 'up' (being up on a topic means being informed about it).
'about' indicates an anagram.
'lions' anagrammed gives 'silon'.
'up'+'silon'='UPSILON'
(Other definitions for upsilon that I've seen before include "Greek character" , "Character from abroad" , "Greek 'u'" , "Greek letter" .)