Where a bighead can get a new hat (4)
I believe the answer is:
bath
'where a bighead' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'can get a new hat' is the wordplay.
'can' becomes 'b' (I can't justify this - if you can you should give a lot more credence to this answer).
'get' means one lot of letters goes inside another.
'a new' is an anagram indicator.
'hat' anagrammed gives 'ath'.
'b' put within 'ath' is 'BATH'.
Can you help me to learn more?
(Other definitions for bath that I've seen before include "It may be run" , "Water container for washing" , "English spa founded by the Romans" , "Large tub" , "Somerset city with abbey and medicinal waters" .)