Guy briefly retained by Touchstone to keep watch (5,5)
I believe the answer is:
stand guard
'keep watch' is the definition.
(I've seen this before)
'guy briefly retained by touchstone' is the wordplay.
'briefly' means to remove the last letter.
'retained by' means one lot of letters goes inside another.
'touchstone' becomes 'standard' (synonyms).
'guy' with its last letter removed is 'gu'.
'gu' put within 'standard' is 'STAND GUARD'.
'to' acts as a link.
(Other definitions for stand guard that I've seen before include "Police" , "Keep watch" , "Act as sentry" .)