At first, scientist loathed awful new guy's jargon (5)

I believe the answer is:

slang

'jargon' is the definition.
(I know that jargon can be written as slang)

'at first scientist loathed awful new guy's' is the wordplay.
'at first' says to take the initial letters.
The first letters of 'scientist loathed awful new guys' is 'SLANG'.

(Other definitions for slang that I've seen before include "Berate" , "See 15" , "abuse" , "Informal language - jargon" , "Any class's jargon" .)

I've seen this clue in The Irish Times.

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