So it's firm cover (3)

I believe the answer is:

lid

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

'cover' is the definition.
(I know that cover can be written as Lid)

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

Can you help me to learn more?

(Other definitions for lid that I've seen before include "Hat" , "Top for bottle or eye protector" , "Removable cover of a container" , "Cap or top for container" , "Receptacle cover" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app