Heard fist was used recklessly (9)

I believe the answer is:

headfirst

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'recklessly' is the definition.
(thesaurus)

'heard fist was' is the wordplay.
'heard fist' anagrams to 'HEADFIRST'.
Though, I am unsure how the anagram is indicated.

'used' is the link.

Can you help me to learn more?

(Other definitions for headfirst that I've seen before include "rashly" , "If trashed, dive as divers usually do" , "Impetuously" , "Without thinking" .)

I've seen this clue in The Mirror.

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