Piece of cake left inside bag (6)
I believe the answer is:
clinch
'bag' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe there's an association between them I don't understand?
'piece of cake left inside' is the wordplay.
'piece of cake' becomes 'cinch' (synonyms).
'left' becomes 'l' (common abbreviation).
'inside' indicates putting letters inside.
'cinch' placed around 'l' is 'CLINCH'.
Can you help me to learn more?
(Other definitions for clinch that I've seen before include "Grapple - confirm" , "seal" , "Matthew ---; --- Schoenberg" , "Passionate embrace" , "Tight (or amorous) embrace" .)