A habit makes us grow old (5)
I believe the answer is:
usage
'a habit' is the definition.
(I've seen this before)
'us grow old' is the wordplay.
'grow old' becomes 'age' (synonyms).
'us'+'age'='USAGE'
'makes' acts as a link.
(Other definitions for usage that I've seen before include "Habitual custom" , "Way of treating" , "Terminology - custom" , "Custom; practice" , "Accepted or habitual practice" .)