Princess dropping ecstasy into poor claret (7)
I believe the answer is:
electra
'princess' is the definition.
(I have seen 'mythological princess' mean 'electra' so perhaps 'princess' could also mean 'electra')
'dropping ecstasy into poor claret' is the wordplay.
'dropping' indicates taking the first letters (I've seen 'drop of' mean this).
'into' indicates putting letters inside.
'poor' is an anagram indicator.
The initial letter of 'ecstasy' is 'e'.
'claret' with letters rearranged gives 'elctra'.
'e' going within 'elctra' is 'ELECTRA'.
(Other definitions for electra that I've seen before include "Mourning Becomes ____ (play cycle Eugene O'Neill)" , "Treacle (anag) - daughter of Agamemnon" , "Agamemnon's daughter" , "Daughter of Agamemnon and Clytemnestra (who has a complex?)" , "Greek heroine (with a complex!)" .)