See some of them swimming in German river (3)
I believe the answer is:
ems
'german river' is the definition.
(I've seen this before)
'see some of them swimming in' is the wordplay.
'see some of' indicates the answer is hidden within the clue.
'EMS' can be found hidden inside 'them swimming in'.
(Other definitions for ems that I've seen before include "Printer's units, each two ens" , "Units of print measure" , "Printers' measures" , "German spa; print measures" , "German city" .)