'Cause you can eat it! (3)
I believe the answer is:
cos
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cos that I've seen before include "Chief of Staff" , "Kind of lettuce in microcosm" , "often associated with sin" , "providing unsatisfactory explanation?" , "healthy food" .)