31 across
Main
movements
of
Mendelssohn’s
4th
introduced
by
strings
(5)
Main movements of Mendelssohn’s 4th introduced by strings (5)
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The definition is at the beginning of the clue.
How the clue works
The definition is Main movements — the movements of the sea, with "main" being a poetic word for the ocean. Now for the wordplay: take strings, which gives you TIES, and "introduce" the fourth letter of Mendelssohn (that's D) into it, slotting it in to get TI-D-ES.