Seethe a short way into series (6)
I believe the answer is:
simmer
'seethe' is the definition.
(both can mean to be angry)
'a short way into series' is the wordplay.
'a short way' becomes 'imm' (resembles 1 mm, a short distance).
'into' means one lot of letters goes inside another.
'series' becomes 'ser' (abbreviation for series).
'imm' put within 'ser' is 'SIMMER'.
(Other definitions for simmer that I've seen before include "Bubble away" , "Cook in gently boiling water" , "Keep just short of boiling point, water or temper" , "Boil slowly at low temperature" , "Gently boil" .)