Change harness covering head (5)

I believe the answer is:

alter

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'change' is the definition.
(I know that change can be written as alter)

I don't understand how the remainder of the clue works.

(Other definitions for alter that I've seen before include "Change or adjust" , "convert" , "Amend" , "reform" , "Revise" .)

I've seen this clue in The Independent.

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