Chewed over half-warmed fish outside gallery (9)
I believe the answer is:
Here is my best explanation:
'chewed over' is the definition.
('meditate' can be a synonym of 'chew over')
'half-warmed fish outside gallery' is the subsidiary indication.
'half-warmed' becomes 'med' (med is half of warmed).
'fish' becomes 'id' (type of fish).
'outside' is an insertion indicator.
'gallery' becomes 'tate'.
'id' enclosing 'tate' is 'itated'.
(Other definitions for meditated that I've seen before include "Engaged in religious contemplation" and "Engaged in spiritual contemplation".)