Departed before improvement from one side? (9)
I believe the answer is:
laterally
'from one side?' is the definition.
'departed before improvement' is the wordplay.
'departed' becomes 'late' (I've seen this before).
'before' is a charade indicator (letters next to each other).
'improvement' becomes 'rally' (recovery after illness or decline).
'late'+'rally'='LATERALLY'
(Other definitions for laterally that I've seen before include "Approaching the side" , "Sideways" .)