Workers sited principally alongside river (5)
I believe the answer is:
staff
'workers' is the definition.
(members of staff)
'sited principally alongside river' is the wordplay.
'principally' suggests taking the first letters.
'alongside' is a charade indicator (letters next to each other).
'river' becomes 'Taff' (river in Wales).
The first letter of 'sited' is 's'.
's'+'taff'='STAFF'
(Other definitions for staff that I've seen before include "men?" , "team?" , "support for notes" , "Employees; sign of office" , "Employees; weapon" .)