Ancestry of old Irish king traced to Guiana, oddly (6)
I believe the answer is:
origin
'ancestry of old irish king' is the definition.
'origin' can be an answer for 'ancestry' (synonyms). I'm unsure of the 'of old irish king' bit.
'traced to guiana oddly' is the wordplay.
'traced' becomes 'ori' (I can't explain this - if you can you should believe this answer much more).
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
The alternate letters of 'guiana' are 'gin'.
'ori'+'gin'='ORIGIN'
Can you help me to learn more?
(Other definitions for origin that I've seen before include "Where X meets Y" , "Source, beginning point" , "Beginning, starting point" , "Source, derivation" , "Beginning (of species?)" .)