Knut's entertaining extremely tiresome couple (4)
I believe the answer is:
item
'couple' is the definition.
(a romantic couple are 'an item')
'knut's entertaining extremely tiresome' is the wordplay.
'knut's' becomes 'I'm' (where the crossword setter is 'Knut').
'entertaining' is an insertion indicator (inviting in, as for dinner).
'extremely' suggests removing the centre (only the extremes of the word are used).
'tiresome' with its middle taken out is 'te'.
'im' enclosing 'te' is 'ITEM'.
(Other definitions for item that I've seen before include "Bit of news to broadcast" , "'One thing, poor mite (4)'" , "Part of a list" , "Individual unit in a list" , "Single unit" .)