Select old drain to be replaced (6)
I believe the answer is:
ordain
'select' is the definition.
(thesaurus)
'old drain to be replaced' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'to be replaced' indicates anagramming the letters.
'drain' anagrammed gives 'rdain'.
'o'+'rdain'='ORDAIN'
(Other definitions for ordain that I've seen before include "Officially order" , "Establish" , "Invest with ministerial authority" , "Make a decree" , "Inroad (anag.)" .)