Portion of Lotto win goes unpaid (5)
I believe the answer is:
owing
'unpaid' is the definition.
(I've seen this before)
'portion of lotto win goes' is the wordplay.
'portion of' indicates a hidden word.
'OWING' is hidden in the letters of 'lotto win goes'.
(Other definitions for owing that I've seen before include "(Of money) outstanding" , "Unpaid" , "Undischarged" , "Unsettled; on account" , "Still outstanding" .)