Old series that's somewhat pithy (6)
I believe the answer is:
orange
'somewhat pithy' is the definition.
I can't judge whether this defines the answer.
'old series' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'series' becomes 'range' (I have seen 'Series of mountains' mean 'range' so perhaps 'series' could also mean 'range').
'o'+'range'='ORANGE'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for orange that I've seen before include "A rainbow colour" , "Dutch royal house" , "Mandarin?" , "Colour of an onager?" , "Order colour" .)