Thanks for admitting the fresh air – it’s heady! (5)
I believe the answer is:
tiara
'thanks' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'admitting the fresh air it's heady' is the wordplay.
'admitting' is an insertion indicator.
'the fresh' indicates an anagram.
'it's heady' becomes 'ta' (I can't justify this - if you can you should give a lot more credence to this answer).
'air' anagrammed gives 'iar'.
'iar' placed within 'ta' is 'TIARA'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for tiara that I've seen before include "Item of jewellery" , "Decorative headgear" , "Jewelled head-ornament" , "Half-crown?" , "Ornamental crescent-shaped headdress or Pope's diadem" .)