Endlessly tired horse ruined speculator (9)
I believe the answer is:
theoriser
'speculator' is the definition.
(I've seen this before)
'endlessly tired horse ruined' is the wordplay.
'endlessly' means to remove the last letter.
'ruined' indicates an anagram.
'tired' with its final letter taken off is 'tire'.
'tire'+'horse'='tirehorse'
'tirehorse' anagrammed gives 'THEORISER'.
(Other definitions for theoriser that I've seen before include "speculator" , "One who has convictions" , "one who speculates" , "One who reasons" , "he thinks" .)