It goes to one's head, and that's fine (5)
I believe the answer is:
tiara
'fine' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'it goes to one's head and that's' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'goes to' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'head' indicates taking the first letters.
'that's' becomes 'ra' (I can't justify this - if you can you should believe this answer much more).
The initial letter of 'and' is 'a'.
't'+'i'+'a'+'ra'='TIARA'
Can you help me to learn more?
(Other definitions for tiara that I've seen before include "Top decoration" , "Great delight" , "Ornamental head band" , "Jewelled crescent-shaped headdress" , "Crescent-shaped headdrsss or Pope's diadem" .)