Caught teal's head in entrance -- quack! (6)
I believe the answer is:
doctor
'quack' is the definition.
(quack can mean a fraudulent doctor)
'caught teal's head in entrance' is the wordplay.
'head' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'entrance' becomes 'door' (door is a kind of entrance).
The first letters of 'caught teals' is 'ct'.
'ct' going into 'door' is 'DOCTOR'.
(Other definitions for doctor that I've seen before include "change" , "Medical worker or holder of highest university degree" , "Adulterate; medic" , "Ship's cook" , "Physician" .)