Knockout with strength (6)
I believe the answer is:
beauty
This clue is a double definition.
'knockout' is the first definition.
(synonyms)
'strength' is the second definition.
(thesaurus)
'with' is the link.
(Other definitions for beauty that I've seen before include "- Salon; - spot" , "A quality pleasing to the senses" , "Excellent specimen" , "Stunner" , "Quality found in the eye of the beholder?" .)