500 sheep getting a drink (4)
I believe the answer is:
dram
'a drink' is the definition.
(small alcoholic drink)
'500 sheep getting' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'sheep' becomes 'ram' (male sheep).
'getting' says to put letters next to each other.
'd'+'ram'='DRAM'
(Other definitions for dram that I've seen before include "shot" , "A wee spirit" , "Small drink of spirits" , "Wee drink" , "Small drink (in Scotland)" .)