Listened to more ambitious rock (7)

I believe the answer is:

boulder

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

'rock' is the definition.
(I know that boulder is a type of rock)

I don't understand the rest of the clue.

(Other definitions for boulder that I've seen before include "Colorado city" , "Big rock" , "Large smooth rounded rock" , "Large smooth mass of rock" , "Large, loose eroded rock" .)

I've seen this clue in The Sun.

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