It's rare for sad ignoramus to dump Romeo (4,4)
I believe the answer is:
blue moon
'it's rare' is the definition.
(as in 'once in a blue moon')
'sad ignoramus to dump romeo' is the wordplay.
'sad' becomes 'blue' (similar in meaning).
'ignoramus' becomes 'moron' (both can mean a stupid person).
'to dump' suggests deleting specific letters.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'moron' with 'r' taken out is 'moon'.
'blue'+'moon'='BLUE MOON'
'for' acts as a link.
(Other definitions for blue moon that I've seen before include "Coloured lunar phenomenon, happens rarely" , "Rodgers and Hart song" , "In which once is very seldom" , "Indicator of rarity" , "Very rare occurrence" .)