Greek in series causes annoyance (7)
I believe the answer is:
chagrin
'annoyance' is the definition.
(I've seen this before)
'greek in series' is the wordplay.
'greek' becomes 'gr'.
'in' means one lot of letters goes inside another.
'series' becomes 'chain' (chain is a kind of series**).
'gr' placed into 'chain' is 'CHAGRIN'.
'causes' is the link.
(Other definitions for chagrin that I've seen before include "Frustration at having failed" , "Embarrassment - rich nag (anag)" , "Humiliation, mortification" , "Strong feeling of embarrassment" , "Annoy" .)