Additional intelligence European leader held to be redundant (10)
I believe the answer is:
extraneous
'additional' is the definition.
(thesaurus)
'intelligence european leader held to be redundant' is the wordplay.
'intelligence' becomes 'nous' (I've seen this before**).
'leader' suggests taking the first letters.
'held' is an insertion indicator.
'to' means one lot of letters go next to another (I've seen this in other clues).
'be redundant' becomes 'extra' ('extra' can be a synonym of 'redundant').
The first letter of 'european' is 'e'.
'nous' going around 'e' is 'neous'.
'neous' after 'extra' is 'EXTRANEOUS'.
(Other definitions for extraneous that I've seen before include "Unrelated, irrelevant" , "Irrelevant; of outside origin" , "Not belonging" , "Irrelevant to the subject" , "Unrelated to the subject" .)