Crossword Genius

My radar must be off this beam (7)

Ross

I believe the answer is:

yardarm

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'this beam' is the definition.
(type of wooden beam on a ship)

'my radar must be off' is the wordplay.
'my radar' can be anagrammed to 'YARDARM'.
However, I'm not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for yardarm that I've seen before include "Either end of a spar on a square-rigger's mast" , "Tapering end of beam hung on mast" , "My radar (anag)" , "Either end of a sail beam" , "that one finds in marina" .)

I've seen this clue in The Telegraph.
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