High-ranking officer left yacht initially in the main (9)
I believe the answer is:
generally
'in the main' is the definition.
(thesaurus)
'high-ranking officer left yacht initially' is the wordplay.
'high ranking officer' becomes 'general' (I've seen this before).
'initially' suggests taking the first letters.
The first letters of 'left yacht' is 'ly'.
'general'+'ly'='GENERALLY'
(Other definitions for generally that I've seen before include "as usual" , "Usually, as a rule" , "In most cases, as a general rule" , "In most cases, usually" , "most of the time" .)