Observes the intrusion and is furious (7)
I believe the answer is:
seethes
'is furious' is the definition.
(I've seen this before)
'observes the intrusion' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'the' is 'eth' which is within the answer.
'observes' could be 'sees' (I've seen this before) and 'sees' is present in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'and' is the link.
Can you help me to learn more?
(Other definitions for seethes that I've seen before include "is boiling" , "gets hot under the collar" , "is angry" , "furious" , "gets agitated" .)