Additional line encapsulating singular piece (6)
I believe the answer is:
morsel
'piece' is the definition.
(I have seen 'Small piece' mean 'morsel' so perhaps 'piece' could also mean 'morsel')
'additional line encapsulating singular' is the wordplay.
'additional' becomes 'more' ('more' can be a synonym of 'additional').
'line' becomes 'l'.
'encapsulating' indicates putting letters inside (I've seen this in other clues).
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'more'+'l'='morel'
'morel' going around 's' is 'MORSEL'.
(Other definitions for morsel that I've seen before include "Titbit" , "Part" , "Tiny piece" , "Small bit of food" , "A small amount of food, just a piece" .)