Work at river locations - they attract (9)
I believe the answer is:
opposites
'they attract' is the definition.
(I've seen this in another clue)
'work at river locations' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'at' is a charade indicator (letters next to each other).
'river' becomes 'Po' (Italian river).
'locations' becomes 'sites' (site is a kind of location).
'op'+'po'+'sites'='OPPOSITES'
(Other definitions for opposites that I've seen before include "counters" , "What proverbially attract" , "said to be attractive" , "Inverses" , "such as the Poles?" .)