Crew on middle of Test river (6)
I believe the answer is:
ganges
'river' is the definition.
(Ganges is an example)
'crew on middle of test' is the wordplay.
'crew' becomes 'gang' ('gang' can be a synonym of 'crew').
'on' is a charade indicator (letters next to each other).
'middle of' means to look at the middle letters.
The central letters of 'test' are 'es'.
'gang'+'es'='GANGES'
(Other definitions for ganges that I've seen before include "Sacred Hindu river in India" , "Sacred Indian river" , "Holy Indian river" , "Asian flower" , "Holy river on India" .)