Substitutes Greek character blocking second pass back (6)
I believe the answer is:
locums
'substitutes' is the definition.
(locum is a kind of substitute)
'greek character blocking second pass back' is the wordplay.
'greek character' becomes 'mu' (letter from the Greek alphabet).
'blocking' indicates putting letters inside.
'second' becomes 's' (common abbreviation).
'pass' becomes 'col' (col is a kind of pass**).
'back' shows that the letters should be reversed in order.
's'+'col'='scol'
'mu' going into 'scol' is 'smucol'.
'smucol' in reverse letter order is 'LOCUMS'.
(Other definitions for locums that I've seen before include "Stand-in doctors" , "Temporary substitutes, doctors say" , "Temporary substitute doctors" , "Temporary substitutes, for doctors perhaps" , "Delegates" .)