Credulous servicemen? (7)

I believe the answer is:

marines

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

'servicemen?' is the definition.
(marine is a kind of serviceman)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for marines that I've seen before include "Troops trained to serve on land or sea" , "Soldiers" , "Naval service" , "forces" , "Seminar for American troops" .)

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.