Check on one foreign character (3)
I believe the answer is:
chi
'foreign character' is the definition.
(Greek letter)
'check on one' is the wordplay.
'check' becomes 'ch' (abbreviation).
'on' is a charade indicator (letters next to each other).
'one' becomes 'I' (Roman numeral).
'ch'+'i'='CHI'
(Other definitions for chi that I've seen before include "Greek X" , "The 22nd letter of Greek alphabet" , "T'ai -, martial art" , "Classical character" , "One character abroad" .)