Maybe harmful chemical upset stomach — a fellow taken short (7)

I believe the answer is:

mutagen

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'maybe harmful chemical' is the definition.
The definition suggests a singular noun which matches the answer.

'upset stomach a fellow taken short' is the wordplay.
'upset' is a reversal indicator.
'stomach' becomes 'tum' (synonyms).
'a fellow taken short' becomes 'agen' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tum' in reverse letter order is 'mut'.
'mut'+'agen'='MUTAGEN'

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(Other definitions for mutagen that I've seen before include "chemical substance?" , "Changing agent" , "Biochemical substance" .)

I've seen this clue in The Times.

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