Fastener sank, ultimately blocking drain (6)
I believe the answer is:
skewer
'fastener' is the definition.
(skewer is a kind of fastener)
'sank ultimately blocking drain' is the wordplay.
'ultimately' indicates one should take the final letters.
'blocking' means one lot of letters goes inside another.
'drain' becomes 'sewer' (sewer is a kind of drain**).
The final letter of 'sank' is 'k'.
'k' placed within 'sewer' is 'SKEWER'.
(Other definitions for skewer that I've seen before include "Spike on which kebab is cooked" , "Run through" , "Pin for cooking meats" , "Stick on which kebab is cooked" , "Long metal pin" .)