Thought first of ladies supreme (5)
I believe the answer is:
ideal
'supreme' is the definition.
(thesaurus)
'thought first of ladies' is the wordplay.
'thought' becomes 'idea' ('idea' can be a synonym of 'thought'**).
'first of' suggests taking the first letters.
The initial letter of 'ladies' is 'l'.
'idea'+'l'='IDEAL'
(Other definitions for ideal that I've seen before include "Optimum" , "Perfect but not likely to become a reality" , "Standard to aim for" , "In her dreams" , "Very eager (to see)" .)