Article Phi’s written up about grand puzzle (6)
I believe the answer is:
enigma
'puzzle' is the definition.
(I've seen this before)
'article phi's written up about grand' is the wordplay.
'article phi' becomes 'amine' (I can't explain this - if you can you should believe this answer much more).
'written up' says the letters should be written backwards.
'about' means one lot of letters goes inside another.
'grand' becomes 'G' (synonyms).
'amine' backwards is 'enima'.
'enima' enclosing 'g' is 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Difficult problem" , "cryptic puzzle!" , "It's mysterious like gamine" , "Mysterious person/thing" , "Title of Elgar's set of variations" .)