Characters remaining when Hammersmith is evacuated (7)

I believe the answer is:

aitches

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

'characters' is the definition.
(aitch is a kind of character)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for aitches that I've seen before include "Letters" , "Two of which are in Hythe" , "They are regularly dropped in the East End" .)

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