Nun working in favourite basket (6)
I believe the answer is:
punnet
'basket' is the definition.
(I know that punnet is a type of basket)
'nun working in favourite' is the wordplay.
'working' indicates an anagram.
'in' indicates putting letters inside.
'favourite' becomes 'pet' (synonyms).
'nun' anagrammed gives 'unn'.
'unn' going into 'pet' is 'PUNNET'.
(Other definitions for punnet that I've seen before include "Source of fruit" , "Strawberry etc. container" , "Little box (of strawberries?)" , "Strawberry basket" , "Small basket for berries" .)