Old drain fixed to end of wall first, say (7)
I believe the answer is:
ordinal
'first say' is the definition.
('first' is an ordinal number)
'old drain fixed to end of wall' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fixed' is an anagram indicator.
'to end of' says to take the final letters.
The final letter of 'wall' is 'l'.
'drain' with letters rearranged gives 'rdina'.
'o'+'rdina'+'l'='ORDINAL'
(Other definitions for ordinal that I've seen before include "For example, first" , "Old Iran (anag)" , "Kind of number defining position in series" , "Number defining a thing's position in series" , "First, second, third could be such a numeral" .)