Regulars in the bar having a drink (3)
I believe the answer is:
tea
'a drink' is the definition.
(tea is a kind of drink)
'regulars in the bar' is the wordplay.
'regulars in' indicates alternate letters.
'the'+'bar'='thebar'
The alternating letters of 'thebar' are 'TEA'.
'having' is the link.
(Other definitions for tea that I've seen before include "Light afternoon repast" , "Evening meal - eat up" , "Perhaps Indian" , "Assam, for instance" , "Beverage from leaves" .)