Kept in trunk, nothing is free (6)
I believe the answer is:
unknot
'free' is the definition.
(thesaurus)
'kept in trunk nothing is' is the wordplay.
'kept in' says the answer is hidden in the clue.
'UNKNOT' is hidden within 'trunk nothing is'.
(Other definitions for unknot that I've seen before include "Simplify" , "Free from ties" , "Loosen" , "Untie" , "Remove tie" .)