They show how to make a mixture of rum and ale (8)
I believe the answer is:
formulae
'they show how to' is the definition.
The definition suggests an adverb but the answer is not.
'a mixture of rum and ale' is the wordplay.
'a mixture' is an anagram indicator.
'and' says to put letters next to each other.
'of'+'rum'='ofrum'
'ofrum' put after 'ale' is 'aleofrum'.
'aleofrum' with letters rearranged gives 'FORMULAE'.
'make' is the link.
Can you help me to learn more?
(Other definitions for formulae that I've seen before include "Recipes" , "See 3" , "Mathematical relationships or rules stated in symbols" , "Tried and tested methods" , "prescriptions" .)