15 across
Measure
English
trout
initially
found
in
French
sea
(5)
Measure English trout initially found in French sea (5)
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The definition is at the beginning of the clue.
How the clue works
Right, so Measure is your definition, pointing to METER. For the wordplay, take MER, the French word for sea, and prise it open — inside goes E for English, then T for trout initially, giving you M-E-T-ER.