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Soldiers overpower informer on the house (6)

Ross

I believe the answer is:

gratis

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)

'soldiers overpower informer' is the wordplay.
I cannot really see how this works, but
'soldiers' could be 'ra' (abbreviation for Royal Artillery) and 'ra' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for gratis that I've seen before include "Free, at no cost" , "Free of charge" , "Without charge" , "Costing nothing, free" , "On the house" .)

I've seen this clue in The Hindu.
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