Serious Scots greeting one half-heartedly (5)

I believe the answer is:

sober

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'serious' is the definition.
(sober can mean sensible or serious)

I can't explain the rest of the clue.

(Other definitions for sober that I've seen before include "Earnest" , "Far from drunk" , "Serious, dull" , "Abstemious - grave" , "Temperate" .)

I've seen this clue in The Sunday Times and The Times.

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