Research facility in Switzerland retains first reservoir (7)
I believe the answer is:
cistern
'reservoir' is the definition.
(I know that cistern is a type of reservoir)
'research facility in switzerland retains first' is the wordplay.
'research facility in switzerland' becomes 'CERN' (CERN, particle physics laboratory in Switzerland).
'retains' means one lot of letters goes inside another.
'first' becomes 'ist' (ist resembles 1st).
'cern' enclosing 'ist' is 'CISTERN'.
(Other definitions for cistern that I've seen before include "Water storage tank" , "Tank, reservoir" , "Water container" , "Vat" , "bit of toilet" .)