Forget to order vermouth (4)
I believe the answer is:
omit
'forget' is the definition.
(thesaurus)
'order vermouth' is the wordplay.
'order' becomes 'om' (order of merit).
'vermouth' becomes 'it' (short for 'Italian vermouth' as in 'gin and it').
'om'+'it'='OMIT'
'to' acts as a link.
(Other definitions for omit that I've seen before include "Take out" , "Fail to use" , "by-pass" , "Don't include" , "Exclude; veto" .)