Beryl starts to enjoy my entertaining repartee at Lady's Day (7)
I believe the answer is:
emerald
'beryl' is the definition.
(thesaurus)
'starts to enjoy my entertaining repartee at lady's day' is the wordplay.
'starts to' suggests taking the first letters.
'day' becomes 'd'.
The first letters of 'enjoy my entertaining repartee at lady' is 'emeral'.
'emeral'+'d'='EMERALD'
(Other definitions for emerald that I've seen before include "La couleur" , "Small size of type, in printing" , "Precious green stone" , "a type employed by the press" , "Bright green colour" .)