Four pairs of volunteers beset by obsessive behaviour (5)
I believe the answer is:
octad
'four pairs' is the definition.
'volunteers beset by obsessive behaviour' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'obsessive behaviour' becomes 'ocd' (obsessive-compulsive disorder).
'ta' put within 'ocd' is 'OCTAD'.
'of' acts as a link.
(Other definitions for octad that I've seen before include "Group" , "Crew, perhaps" , "E.g. quarterfinalists" , "quarter-finalists, perhaps" .)