Chewed over half-warmed fish outside gallery (9)
I believe the answer is:
meditated
'chewed over' is the definition.
('meditate' can be a synonym of 'chew over')
'half-warmed fish outside gallery' is the wordplay.
'half-warmed' becomes 'med' (med is half of warmed).
'fish' becomes 'id' (type of fish).
'outside' means one lot of letters goes inside another.
'gallery' becomes 'tate' (I've seen this before).
'id' going around 'tate' is 'itated'.
'med'+'itated'='MEDITATED'
(Other definitions for meditated that I've seen before include "Engaged in spiritual contemplation" , "pondered" , "Engaged in religious contemplation" , "thought" .)