Prolific detective sounds like he could become gloomy (7)

I believe the answer is:

maigret

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

'prolific detective' is the definition.
'maigret' can be an answer for 'detective' (I've seen this before). I'm not certain of the 'prolific' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for maigret that I've seen before include "Fictional detective" , "Policeman" , "Simenon''s detective-superintendent" , "cracker?" , "Georges Simenon's detective" .)

I've seen this clue in The Guardian.

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.