Boat loaded with most of new basket (6)
I believe the answer is:
Here is my best explanation:
'basket' is the definition.
(punnet is a kind of basket)
'boat loaded with most of new' is the wordplay.
'boat' becomes 'punt' (I've seen this before).
'loaded with' means one lot of letters goes inside another.
'most of' means to remove the last letter.
'new' with its last letter taken off is 'ne'.
'punt' placed around 'ne' is 'PUNNET'.
(Other definitions for punnet that I've seen before include "Light basket for fruit", "Small light basket, for strawberries say", "Fruit 18 Down", "Little box (of strawberries?)", "Small basket for berries", "Small light basket for fruit", "source", "A small basket for berries", "Small basket for 13 Across (6)" and "Source of fruit".)