George Hartley confused about lassitude (8)
I believe the answer is:
lethargy
'lassitude' is the definition.
(synonyms)
'george hartley confused about' is the wordplay.
'george' becomes 'g'.
'confused' indicates an anagram.
'about' indicates putting letters inside.
'hartley' anagrammed gives 'lethary'.
'g' going within 'lethary' is 'LETHARGY'.
(Other definitions for lethargy that I've seen before include "Weariness" , "Lassitude, sluggishness" , "Drowsy" , "Sloth" , "'Lassitude, langour (8)'" .)