The line of a cooperative former monarch (6)

I believe the answer is:

cooper

I'm a little stuck... Click here to teach me more about this clue!

'the line' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'cooperative former monarch' is the wordplay.
'cooperative' becomes 'co-op' (synonyms).
'former monarch' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
'coop'+'er'='COOPER'

'of a' is the link.

Can you help me to learn more?

(Other definitions for cooper that I've seen before include "Craftsman" , "He makes and repairs barrels" , "Cask and barrel maker" , "Maker of barrels" , "Porter with stout" .)

I've seen this clue in the Evening Standard.

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.