Grow old, only occasionally well-behaved? (4)
I believe the answer is:
good
'well-behaved?' is the definition.
('good' can be similar in meaning to 'well-behaved')
'grow old only occasionally' is the wordplay.
'only occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'grow'+'old'='growold'
The alternate letters of 'growold' are 'GOOD'.
(Other definitions for good that I've seen before include "promoting welfare" , "Fine" , "Well done! Health-giving" , "Priestley's companions" , "Virtuous - valid" .)