Rage definitely gets old (4)
I believe the answer is:
aged
'old' is the definition.
('aged' can be similar in meaning to 'old')
'rage definitely gets' is the wordplay.
'gets' indicates the answer is hidden within the clue.
'AGED' is hidden within 'rage definitely'.
(Other definitions for aged that I've seen before include "Old or ancient" , "way over the hill" , "Time-worn" , "Very old, person or wine" , "Grew old" .)