A glue has spilled on the top end, which is a nuisance (6)
I believe the answer is:
plague
'a nuisance' is the definition.
(thesaurus)
'a glue has spilled on the top end' is the wordplay.
I cannot really understand how this works, but
'a' is found in the answer.
an anagram of 'glue' is 'lgue' which is located in the remaining letters.
A single letter 'p' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'which is' acts as a link.
Can you help me to learn more?
(Other definitions for plague that I've seen before include "Swarm of insects" , "Outbreak of a deadly disease, or infestation of pest like locusts" , "Widespread, contagious and destructive disease" , "Large-scale illness" , "Badger" .)