He's adrift in panic having gone in precipitately (9)

I believe the answer is:

headfirst

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'precipitately' is the definition.
(synonyms)

'he's adrift in panic having gone' is the wordplay.
'he's adrift' can be anagrammed to 'HEADFIRST'.
However, I'm not clear how this is indicated.

'in' acts as a link.

Can you help me to learn more?

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

I've seen this clue in The Globe and Mail.

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