Old lady supporting one in English mystery (6)
I believe the answer is:
enigma
'mystery' is the definition.
(synonyms)
'old lady supporting one in english' is the wordplay.
'old lady' becomes 'ma'.
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'one' becomes 'I' (Roman numeral).
'in' means one lot of letters goes inside another.
'english' becomes 'eng' (abbreviation).
'i' going into 'eng' is 'enig'.
'ma' after 'enig' is 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Difficult problem" , "Puzzle found in gem, Ian" , "Mysterious person/thing" , "Gamine being a bit of a puzzle" , "There's a mystery in game" .)