Say yes to account, with 100 European banks in protest (6)
I believe the answer is:
accept
'say yes to' is the definition.
(I've seen this before)
'account with 100 european banks in protest' is the wordplay.
'account' becomes 'ac'.
'with' says to put letters next to each other.
'100' becomes 'c' (roman numeral).
'european' becomes 'e' (abbreviation e.g. EU).
'banks in' means to remove the middle letters (the edges or banks of the word).
'protest' with its centre removed is 'pt'.
'ac'+'c'+'e'+'pt'='ACCEPT'
(Other definitions for accept that I've seen before include "Take up, obtain" , "Reply in the affirmative" , "Receive voluntarily" , "one wouldn't say no to this" , "Welcome" .)