Great joy when sister, say, dismisses Romeo (7)
I believe the answer is:
elation
'great joy' is the definition.
(I've seen this before)
'sister say dismisses romeo' is the wordplay.
'sister say' becomes 'relation' (sister is a kind of relation).
'dismisses' is a deletion indicator.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'relation' with 'r' taken away is 'ELATION'.
'when' acts as a link.
(Other definitions for elation that I've seen before include "Ecstasy -- toenail (anag)" , "High feeling?" , "Delight, thrill" , "Great happiness and exhilaration" , "There's great joy" .)