The most common and most likely answer to this crossword clue is the 8 letter word CASEMENT
I've seen this crossword clue in Financial Times
C
A
S
E
M
E
N
T
More clues leading to the result CASEMENT
- Seeing that put in to reinforce a window
- Join guards when finding Irish nationalist
- “Roger”, as in building material
- Setter gasping a small feature of Windows
- Window has a small sticker on the outside
- Adhesive holds as window opening sideways
- Window when set in mortar
- Irish Roger and English fellows in list of actors
- With carbon on top, black must be scrubbed from lower-storey window
- Patient workers with time frame for window
Buddy explains!

The straight part
We know that "Irish nationalist" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case irish nationalist = casement.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 CASEMENT mean?
CASEMENT noun- A window sash that is hinged on the side.
- A window having such sashes; a casement window.Wp
- Occasionally seen as a usage error due to the similarity of the words: A casemate.