The one that's right (3)
I believe the answer is:
yea
'right' is the definition.
(thesaurus)
'the one' is the wordplay.
I cannot quite see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is found within the answer.
The remaining letters 'ye' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'that's' is the link.
Can you help me to learn more?
(Other definitions for yea that I've seen before include "Emphatic agreement" , "Affirmative in the British Parliament" , "Certainly (archaic)" , "-- or nay?" , "indeed" .)