Principal spread out without thought (8)

I believe the answer is:

headlong

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'without thought' is the definition.
I can't tell whether this defines the answer.

'principal spread' is the wordplay.
'principal' becomes 'head' (synonyms).
'spread' becomes 'long' (I am not sure about this - if you are sure you should believe this answer much more).
'head'+'long'='HEADLONG'

'out' is the link.

Can you help me to learn more?

(Other definitions for headlong that I've seen before include "In hasty or plunging manner" , "Precipitate (flight)" , "Rashly" , "Impetuous" , "Hasty and rash, falling or plunging" .)

I've seen this clue in The Independent.

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