Tees had the ne plus ultra , ensnaring old fool (5)
I believe the answer is:
idiot
'fool' is the definition.
(synonyms)
'tees had the ne plus ultra ensnaring old' is the wordplay.
'tees had' becomes 'I'd' (where the crossword setter is 'Tees').
'the ne plus ultra' becomes 'it' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ensnaring' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'it' placed around 'o' is 'iot'.
'id'+'iot'='IDIOT'
Can you help me to learn more?
(Other definitions for idiot that I've seen before include "nutter" , "The - (Dostoievsky)" , "Dunce" , "'The -----'. novel by Dostoyevsky (1868)" , "Dipstick" .)