Only a long-headed individual (5)

I believe the answer is:

alone

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'only' is the definition.
(synonyms)

'a long-headed individual' is the wordplay.
'long-headed individual' becomes 'lone' (I can't justify this - if you can you should believe this answer much more).
'a'+'lone'='ALONE'

Can you help me to learn more?

(Other definitions for alone that I've seen before include "Isolated from others" , "Singly" , "without partner?" , "Without others present" , "Unaccompanied" .)

I've seen this clue in the Evening Standard.

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