Crossword Genius

Initially special service (3)

Ross

I believe the answer is:

sas

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'special service' is the definition.
(Special Air Service)

I can't explain the rest of the clue.

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(Other definitions for sas that I've seen before include "Elite fighting force" , "Commando-type unit of the British army" , "Specialist regiment of the British army, in short" .)

I've seen this clue in the Evening Standard.
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