17 across
Early
sub
detector
regularly
seen
near
us
in
Dominica
(5)
Early sub detector regularly seen near us in Dominica (5)
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The definition is at the beginning of the clue.
How the clue works
The definition is Early sub detector — ASDIC being that old sonar system for spotting submarines. Now, regularly seen is doing the heavy lifting: take every third letter of near us in Dominica giving A, S, D, I, C.