Swimming tutor upset about a bighead (6)
I believe the answer is:
turbot
'swimming' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'tutor upset about a bighead' is the wordplay.
'upset' is an anagram indicator.
'about' means one lot of letters goes inside another.
'a bighead' becomes 'b' (the head letter of 'big').
'tutor' anagrammed gives 'turot'.
'turot' going around 'b' is 'TURBOT'.
Can you help me to learn more?
(Other definitions for turbot that I've seen before include "Large food fish" , "Rub tot out with the fish" , "Edible flatfish" , "Species of flatfish" , "Flatfish - trout B (anag)" .)