Petty officers fixed the drink (6)
I believe the answer is:
posset
'the drink' is the definition.
(posset is a kind of drink)
'petty officers fixed' is the wordplay.
'petty officers' becomes 'pos' (I am not sure about this - if you are sure you should believe this answer much more).
'fixed' becomes 'set' (both can mean decided or determined).
'pos'+'set'='POSSET'
Can you help me to learn more?
(Other definitions for posset that I've seen before include "As relief for cold" , "Curdled milk drink" , "Sweet spiced milk curdled with alcohol" , "So step on curdled milk once used as a cold remedy" .)