Test one not up and seek to deceive (3,2,2)
I believe the answer is:
try it on
'test' is the definition.
(thesaurus)
'one not up and seek to deceive' is the wordplay.
I cannot really understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
an anagram of 'not' is 'ton' which is within the answer.
'seek' could be 'try' (synonyms) and 'try' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for try it on that I've seen before include "Be audacious" , "Be cheeky" , "Aim to deceive" , "Attempt to be provocative (colloq.)" , "be bold" .)