A custom that makes us go grey (5)
I believe the answer is:
usage
'a custom' is the definition.
('usage' can be a synonym of 'custom')
'us go grey' is the wordplay.
'go grey' becomes 'age' (a person's hair often goes grey as they age).
'us'+'age'='USAGE'
'that makes' acts as a link.
(Other definitions for usage that I've seen before include "Custom - treatment" , "Tradition - rate of consumption" , "Usual practice" , "Silly little thing" , "Acceptable language" .)