Candidate one ordered to guard quarry (7)
I believe the answer is:
nominee
'candidate' is the definition.
(I know that candidate can be written as nominee)
'one ordered to guard quarry' is the wordplay.
'ordered' indicates anagramming the letters.
'to guard' indicates putting letters inside.
'quarry' becomes 'mine' (type of mine).
'one' is an anagram of 'noe'.
'noe' going around 'mine' is 'NOMINEE'.
(Other definitions for nominee that I've seen before include "Person put forward" , "Candidate for office" , "Chosen person" , "One named as candidate" , "One up for a job" .)