Saw knees unstable because of poor condition (8)
I believe the answer is:
weakness
'condition' is the definition.
(I know that weakness is a type of ill luck)
'saw knees unstable' is the wordplay.
'unstable' is an anagram indicator.
'saw'+'knees'='sawknees'
'sawknees' anagrammed gives 'WEAKNESS'.
'because of poor' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for weakness that I've seen before include "Infirmity" , "Flaw, failing" , "Achilles heel" , "Frailty - flaw" , "Fault or flaw" .)