Employment makes us get older (5)
I believe the answer is:
usage
'employment' is the definition.
(I know that employment can be written as usage)
'us get older' is the wordplay.
'get older' becomes 'age' (synonyms).
'us'+'age'='USAGE'
'makes' acts as a link.
(Other definitions for usage that I've seen before include "Custom - treatment" , "Usual practice" , "Tradition - rate of consumption" , "Way of treating" , "Habitual custom" .)