Saw knees trembling in passion? (8)
I believe the answer is:
weakness
'passion?' is the definition.
(thesaurus)
'saw knees trembling' is the wordplay.
'trembling' indicates anagramming the letters.
'saw'+'knees'='sawknees'
'sawknees' is an anagram of 'WEAKNESS'.
'in' acts as a link.
(Other definitions for weakness that I've seen before include "Failing or powerlessness" , "Fault or flaw" , "Frailty - flaw" , "Defect" , "Achilles heel" .)