Cave man of Gaelic legend (6)

I believe the answer is:

fingal

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'cave man of gaelic legend' is the definition.
The definition and answer are not the same part of speech. However, I know that 'cave' relates to this answer.

'cave man' is the wordplay.
'cave' becomes 'fin' (I can't justify this - if you can you should believe this answer much more).
'man' becomes 'gal'.
'fin'+'gal'='FINGAL'

Can you help me to learn more?

(Other definitions for fingal that I've seen before include "Caveman" , "His cave is on Staffa" , "Epic Scot" .)

I've seen this clue in the Evening Standard.

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