25 across
All
together
now
–
express
disappointment
at
what
comes
before
do
(5)
All together now – express disappointment at what comes before do (5)
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The definition is at the beginning of the clue.
How the clue works
"All together now" is your definition there, pointing to the musical term for everyone playing at once. For the wordplay, start with TUT (that's your expression of disappointment) then add TI, since TI is the note that comes right before DO in the musical scale.