Sadly tie knots at a constant rate (8)
I believe the answer is:
steadily
'at a constant rate' is the definition.
(at a constant or steady pace)
'sadly tie knots' is the wordplay.
'knots' is an anagram indicator (I've seen 'knotted' mean this).
'sadly'+'tie'='sadlytie'
'sadlytie' is an anagram of 'STEADILY'.
(Other definitions for steadily that I've seen before include "Regularly, without fluctuation" , "At a regular pace" , "with regularity" , "Daily set (anag)" , "Unfalteringly; gradually" .)