Knut's punched doctor who's always tanked up at work (5)
I believe the answer is:
diver
'up at work' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'knut's punched doctor who's always tanked' is the wordplay.
I cannot really see how this works, but
'knut's' could be 'ive' (where the crossword setter is 'Knut') and 'ive' is located in the answer.
'doctor' could be 'dr' (abbreviation) and 'dr' is located in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for diver that I've seen before include "One plunges head first into water" , "Bird that plunges" , "Grebe maybe" , "Water bird" , "Underwater worker" .)