When ill in quarters, ordered a sedative (13)

I believe the answer is:

tranquilliser

'sedative' is the definition.
(I know that tranquilliser is a type of sedative-hypnotic)

'when ill in quarters ordered a' is the wordplay.
'ill in quarters' anagrams to 'TRANQUILLISER'.
However, I am not clear how the anagram is indicated.

(Other definitions for tranquilliser that I've seen before include "Calming drug" , "A number" , "Pill used to calm" , "Calming-down drug" , "Calmer" .)

I've seen this clue in The Guardian.

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