Being teetotal could be boring (3)
I believe the answer is:
dry
I believe this clue is a double definition.
'being teetotal' is the first definition.
('dry' can be a synonym of 'teetotal')
'be boring' is the second definition.
(thesaurus)
'could' is the link.
(Other definitions for dry that I've seen before include "Lacking moisture" , "far from sweet" , "Remove moisture" , "Thirsty" , "Ironic" .)