Cryptic crossword clue
Old song heard in the seat of Westminster, possibly? 11,3

The most common and most likely answer to this crossword clue is the 11 plus 3 letter word LONDONDERRY AIR

I've seen this crossword clue in Financial Times
L
O
N
D
O
N
D
E
R
R
Y
A
I
R

Buddy explains!

The straight part
We know that "Old song" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old song = londonderry air.
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.

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