12 across
Blocks
German
agreement
most
regularly
ignored
(4)
Blocks German agreement most regularly ignored (4)
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The definition is at the beginning of the clue.
How the clue works
Think of **Blocks** as the definition here, as in "clogs up". For the wordplay, take the German word for agreement, JA (that's "yes" to you and me), then add the "most regularly ignored" bit: take every other letter of MOST, skipping the rest, to get MS.