Ahead in the first half of the series, which is great (5)
I believe the answer is:
super
'is great' is the definition.
(both can mean excellent)
'ahead in the first half of the series' is the wordplay.
'ahead' becomes 'up' (similar in meaning).
'in the' is an insertion indicator.
'first half of' suggests halving the letters.
'series' cut in half is 'ser'.
'up' placed into 'ser' is 'SUPER'.
'which' acts as a link.
(Other definitions for super that I've seen before include "Topnotch, terrific" , "Delightful" , "Incomparable" , "Exceptional" , "(Of product) of extra fine quality" .)