Solid, firm (7)

I believe the answer is:

compact

I believe this clue is a double definition.

'solid' is the first definition.
(thesaurus)

'firm' is the second definition.
(similar in meaning)

(Other definitions for compact that I've seen before include "Small and neat - agreement" , "Make denser, agreement" , "Dense; treaty" , "'Condensed, portable (7)'" , "Dense; agreement" .)

I've seen this clue in The Mirror.

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