In Thurso be remarkably clear-headed (5)
I believe the answer is:
sober
'clear-headed' is the definition.
(I've seen this before)
'in thurso be remarkably' is the wordplay.
'in' indicates a hidden word.
'SOBER' is hidden within 'thurso be remarkably'.
(Other definitions for sober that I've seen before include "Abstemious - grave" , "Staid; clear-headed" , "Temperate" , "In control of one's faculties" , "Judge said to be so" .)