A battle with leader in Europe? That's wise (5)
I believe the answer is:
aware
'wise' is the definition.
(thesaurus)
'a battle with leader in europe?' is the wordplay.
'battle' becomes 'war' (I've seen this before**).
'with' says to put letters next to each other.
'leader in' indicates taking the first letters.
The initial letter of 'europe' is 'e'.
'a'+'war'+'e'='AWARE'
'that's' acts as a link.
(Other definitions for aware that I've seen before include "Alert, conscious of" , "Attentive" , "Cognisant, mindful" , "Knowing (of something)" , "Conscious or having knowledge of" .)