It's easy to pick a piece of cake (3-7,5)

'cake' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's a link between them I don't understand?

'it's easy to pick a piece of' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is found within the answer.
'piece' could be 'r' (abbreviation for Rook) and 'r' is present in the answer.
'of' could be 'o' and 'o' is located in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?