Irvine __, author of Trainspotting (5)

I believe the answer is:

welsh

I believe this clue is a double definition.

'irvine' is the first definition.
(I know that irvine family name Welsh)

'author of trainspotting' is the second definition.
(I know that Trainspotting author Welsh)

(Other definitions for welsh that I've seen before include "British language" , "From Swansea, say?" , "Native to, eg, Cardiff" , "From Aberystwyth, for instancewelshw" , "Default on bet in Wales?" .)

I've seen this clue in The Guardian.

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