Scottish runner first to try marijuana (5)

I believe the answer is:

tweed

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'scottish runner' is the definition.
'tweed' can be an answer for 'runner' (river in Scotland). I'm not sure about the 'scottish' bit.

'first to try marijuana' is the wordplay.
'first to' suggests taking the first letters.
'marijuana' becomes 'weed' (synonyms).
The first letter of 'try' is 't'.
't'+'weed'='TWEED'

Can you help me to learn more?

(Other definitions for tweed that I've seen before include "Woollen material" , "Flow" , "Sort of jacket" , "Scottish/English river" , "Rough material" .)

I've seen this clue in The Independent.

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