Jack having a drop of rum, and glass of beer (3)
I believe the answer is:
jar
'glass of beer' is the definition.
(I've seen this before)
'jack having a drop of rum' is the wordplay.
'jack' becomes 'j' (playing card abbreviation).
'having' is a charade indicator (letters next to each other).
'drop of' indicates taking the first letters.
The first letter of 'rum' is 'r'.
'j'+'a'+'r'='JAR'
'and' acts as a link.
(Other definitions for jar that I've seen before include "Pint glass of beer, perhaps" , "Enduring courage" , "Wide-mouthed container" , "Glass of beer (colloq.)" , "Vessel; sudden impact" .)