Passes work with eager singletons primarily (4)
I believe the answer is:
goes
'passes' is the definition.
('go' can be a synonym of 'pass')
'work with eager singletons primarily' is the wordplay.
'work' becomes 'go' (synonyms).
'with' is a charade indicator (letters next to each other).
'primarily' indicates taking the first letters.
The first letters of 'eager singletons' is 'es'.
'go'+'es'='GOES'
(Other definitions for goes that I've seen before include "Travels; attempts" , "Works" , "See 4" , "Moves" , "Departs; operates" .)