Policemen get older? It’s a measure of things! (7)
I believe the answer is:
yardage
'a measure of things' is the definition.
'policemen get older?' is the wordplay.
'policemen' becomes 'yard' (Scotland Yard is the Metropolitan Police HQ).
'get older?' becomes 'age' ('age' can be a synonym of 'get older').
'yard'+'age'='YARDAGE'
'it's' acts as a link.
(Other definitions for yardage that I've seen before include "Distance gained (Am. football)" , "charge for using court" , "use of enclosed space" , "measure" , "What a caddy should know" .)