Using last drops of original Guinness bottles (5)
I believe the answer is:
vials
'bottles' is the definition.
(vial is a kind of bottle)
'using last drops of original guinness' is the wordplay.
'using' becomes 'via' (I've seen this in another clue).
'last drops of' suggests the final letters.
The final letters of 'original guinness' are 'ls'.
'via'+'ls'='VIALS'
(Other definitions for vials that I've seen before include "Vessels" , "Ampoules" , "Small bottle that contains a drug" , "Small bottles of medicine" , "Small glass containers" .)