Commission old drain layout (6)
I believe the answer is:
ordain
'commission' is the definition.
(thesaurus)
'old drain layout' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'layout' indicates anagramming the letters.
'drain' anagrammed gives 'rdain'.
'o'+'rdain'='ORDAIN'
(Other definitions for ordain that I've seen before include "Elevate or appoint to priesthood" , "Decree and admit to holy orders" , "Lay down (law)" , "Make someone a clergyman" , "Invest with priestly authority" .)