A fluid, but methinks not writing fluid (5)

I believe the answer is:

meths

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'a fluid but' is the definition.
I can't tell whether this definition defines the answer.

'methinks not writing fluid' is the wordplay.
'not' is a deletion indicator.
'writing fluid' becomes 'ink' (fluid used for writing).
'methinks' with 'ink' taken out is 'METHS'.

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(Other definitions for meths that I've seen before include "Methylated spirits (colloq.)" , "Violet-coloured alcohol" , "Fuel" , "Spirit" , "unsuitable alcohol" .)

I've seen this clue in the Evening Standard.

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