Detective’s clothes are so simple (5)

I believe the answer is:

plain

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'simple' is the definition.
(synonyms)

'detective's clothes are' is the wordplay.
'detective's clothes' becomes 'p' (I can't explain this - if you can you should believe this answer much more).
'are' becomes 'lain' (lying is a kind of being).
'p'+'lain'='PLAIN'

'so' acts as a link.

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(Other definitions for plain that I've seen before include "Unattractive - flat" , "Unadorned; evident" , "Unprepossessing" , "Not elaborate; clear" , "Unadorned on flat area" .)

I've seen this clue in The Times.

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