Temperature in reservoirs is baffling (6)
I believe the answer is:
stumps
'is baffling' is the definition.
(stump can mean to confuse or stump)
'temperature in reservoirs' is the wordplay.
'temperature' becomes 't' (abbreviation).
'in' indicates putting letters inside.
'reservoirs' becomes 'sumps' (sump is a kind of reservoir).
't' placed inside 'sumps' is 'STUMPS'.
(Other definitions for stumps that I've seen before include "Baffles on the cricket pitch" , "Baffles -- projecting remnants" , "in match, finally drawn" , "(In cricket) close of play" , "Dismisses batsman" .)