Setter, 'orrible character (4)

I believe the answer is:

agar

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'setter' is the definition.
(I've seen this before)

''orrible character' is the wordplay.
''orrible' becomes 'aga' (I can't justify this - if you can you should believe this answer much more).
'character' becomes 'r'.
'aga'+'r'='AGAR'

Can you help me to learn more?

(Other definitions for agar that I've seen before include "part of culture?" , "Food-thickening jelly" , "What comes out of seaweed" , "Jelly got from seaweeds, used in foods" , "Jelly used in bacteria culture" .)

I've seen this clue in The Times.

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