25 across
Pompous
Unionist
blocking
a
timid
one,
mostly?
That’s
of
doubtful
value
(9)
Pompous Unionist blocking a timid one, mostly? That’s of doubtful value (9)
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The definition is at the end of the clue.
How the clue works
Right, so the definition's sitting at the end there — That's of doubtful value, giving you something uncertain or unclear. For the wordplay: take A plus MOUSE (your "timid one"), then chop it down "mostly" to get AMOUS. Now stuff BIG (that's "pompous") and U (Unionist) into the middle of it, and you get AM-BIG-U-OUS slotting together nicely.