20 down
Slim
line
in
transmitter
(7)
Slim line in transmitter (7)
hints
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checks
0
hints
The definition is at the beginning of the clue.
How the clue works
Look at Slim — that's your definition. You've got SENDER, meaning a transmitter, and you slot in L (the abbreviation for line) after the first letter, splitting it into S and ENDER to give you S-L-ENDER.