Ache left with a poisoned wound is not going away quickly (4-7)

I believe the answer is:

long-lasting

'not going away quickly' is the definition.
(lasting a long time)

'ache left with a poisoned wound' is the wordplay.
'ache' becomes 'long' (synonyms).
'left' becomes 'l' (common abbreviation).
'with' says to put letters next to each other.
'poisoned wound' becomes 'sting' (a sting contains poison).
'long'+'l'+'a'+'sting'='LONG-LASTING'

'is' is the link.

(Another definition for long-lasting that I've seen is " Durable".)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.