Crossword Genius

Like flat battery on the house! (4,2,6)

Ross

I believe the answer is:

free of charge

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

'on the house' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for free of charge that I've seen before include "On the house" , "Gratis" .)

I've seen this clue in the Sydney Morning Herald.
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