Writer of sad poems ties leg in knots (7)
I believe the answer is:
elegist
'writer of sad poems' is the definition.
(I've seen this in another clue)
'ties leg in knots' is the wordplay.
'in knots' indicates an anagram (I've seen this in other clues).
'ties'+'leg'='tiesleg'
'tiesleg' with letters rearranged gives 'ELEGIST'.
(Other definitions for elegist that I've seen before include "Writer of mournful verse" , "bemoaning linesman" , "Gloomy poet" , "Kind of writer" , "Mournful poet" .)