Cook tries open sandwich (9)
I believe the answer is:
interpose
'sandwich' is the definition.
(thesaurus)
'cook tries open' is the wordplay.
'cook' indicates anagramming the letters.
'tries'+'open'='triesopen'
'triesopen' with letters rearranged gives 'INTERPOSE'.
(Other definitions for interpose that I've seen before include "Place between" , "Insert between (one thing and another)" , "Muscle in" , "Intervene between parties" , "mediate" .)