One thing in another — that’s the answer (8)
I believe the answer is:
solution
'answer' is the definition.
(synonyms)
'one thing in another that's the' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'another' could be 'o' (I've seen this in other clues) and 'o' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for solution that I've seen before include "Liquid mixture of two or more substances" , "you're currently seeking?" , "Drink" , "Elucidation - mixture of substances" , "Homogeneous mixture" .)