Welcome rule to get old and mean (7)
I believe the answer is:
average
'mean' is the definition.
(I know that mean can be written as average)
'welcome rule to get old' is the wordplay.
'welcome' becomes 'ave' (I've seen this before).
'rule' becomes 'r' (legal abbreviation).
'to get old' becomes 'age' (to age is to get older).
'ave'+'r'+'age'='AVERAGE'
'and' acts as a link.
(Other definitions for average that I've seen before include "Arithmetically ordinary?" , "Arithmetic mean" , "Mean - typical" , "Medium; normal" , "par" .)