The most common and most likely answer to this crossword clue is the 11 letter word EUROSCEPTIC
I've seen this crossword clue in Financial Times
E
U
R
O
S
C
E
P
T
I
C
More clues leading to the result EUROSCEPTIC
- Hater of Brussels sprouts to start with mushroom, having obsessed about removing cap
- Cup tie score upset Brussels fan? No!
- Cup tie score excited one not keen on Union
- Court pieces excited one not wanting to devolve more power
- Lacking community spirit, warped or cut pieces
- Maybe Leave voter's awfully poetic curse
- . Secret coup I organised, being fixated on sovereignty
- Boris Johnson is one in pieces, upset with court
- Whose reservations on the continent may be confirmed
- Doubter upset soccer tie-up
Buddy explains!

The straight part
We know that "Whose reservations on the continent may be confirmed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case whose reservations on the continent may be confirmed = eurosceptic.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does EUROSCEPTIC mean?
EUROSCEPTIC noun- One who is sceptical of European integration, such as the EEC, the EC, the EP, the EU, the Euro, etc.