Gradually impair dress in blue (4,4)

I believe the answer is:

wear down

'gradually impair' is the definition.

'dress in blue' is the wordplay.
'dress in' becomes 'wear' (I've seen this before).
'blue' becomes 'down' (I've seen this before).
'wear'+'down'='WEAR DOWN'

(Another definition for wear down that I've seen is " Gradually erode".)

I've seen this clue in The Times.

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