Cave man who got a duplicated letter from Fine Gael (6)

I believe the answer is:
fingal
'cave man who got a duplicated' is the definition.
The answer and definition are not the same part of speech. However, I know that 'cave' relates to this answer.
'letter from fine gael' is the wordplay.
'letter' becomes 'f'.
'from' becomes 'in' (I've seen this in other clues).
'fine gael' becomes 'gal' (I am not sure about this - if you are sure you should believe this answer much more).
'f'+'in'+'gal'='FINGAL'
Can you help me to learn more?
(Other definitions for fingal that I've seen before include "His cave is on Staffa" , "Epic Scot" , "Caveman" .)
