River sample from retired bacteriologist (5)
I believe the answer is:
loire
'river' is the definition.
(Loire is an example)
'sample from retired bacteriologist' is the wordplay.
'retired' is a reversal indicator.
'sample from' indicates a hidden word (a sample of letters from the clue).
'eriol' can be found hidden inside 'bacteriologist'.
'eriol' back-to-front is 'LOIRE'.
(Other definitions for loire that I've seen before include "River of central France" , "France's longest river" , "French runner" , "flower found amid vineyards" , "Longest river in France" .)