Pop act of the '80s, The Firm? (7)

I believe the answer is:

adamant

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'the firm?' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for adamant that I've seen before include "Unswerving" , "Positive or intransigent" , "Unyielding, won't change mind" , "intractable" , "Obdurate and unyielding" .)

I've seen this clue in The Everyman puzzle in The Observer.

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