Left over quail prepared by colonel without formality (10)
I believe the answer is:
colloquial
'without formality' is the definition.
(colloquial language is informal)
'left over quail prepared by colonel' is the wordplay.
'left over' becomes 'lo' (this could be a standard abbreviation I've not previously seen).
'prepared' is an anagram indicator.
'by' says to put letters next to each other.
'colonel' becomes 'col'.
'quail' with letters rearranged gives 'quial'.
'lo'+'quial'='loquial'
'loquial' put after 'col' is 'COLLOQUIAL'.
(Other definitions for colloquial that I've seen before include "Used in everyday conversation" , "Used in common conversation" , "Using informal conversation" .)