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 one could define the other.
'knut's punched doctor who's always tanked' is the wordplay.
I cannot really understand 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 present in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
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" .)