Mean greeting from Roman? Be angry (7)
I believe the answer is:
average
'mean' is the definition.
(I know that mean can be written as average)
'greeting from roman? be angry' is the wordplay.
'greeting from roman?' becomes 'ave' (Latin greeting).
'be angry' becomes 'rage' (to rage is to be angry about something).
'ave'+'rage'='AVERAGE'
(Other definitions for average that I've seen before include "Mean figure?" , "Mean; standard" , "About normal or usual" , "par" , "Arithmetical mean" .)