Port Authority in the clear (5)

I believe the answer is:

plain

'the clear' is the definition.
(thesaurus)

'port authority in' is the wordplay.
'port authority' becomes 'pla' (Port of London Authority).
'pla'+'in'='PLAIN'

(Other definitions for plain that I've seen before include "lacking special beauty" , "Obvious - ordinary" , "Ordinary; flat region" , "Clear - open - grassland" , "Ordinary; area of land" .)

I've seen this clue in the Evening Standard.

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