Water in moderation (4)
I believe the answer is:
'water' is the definition.
(Oder is an example)
'in moderation' is the wordplay.
'in' indicates a hidden word.
'ODER' can be found hidden inside 'moderation'.
(Other definitions for oder that I've seen before include "flowing water" , "German and Polish river" , "River that, with the Neisse, forms the German-Polish border" , "European banker" , "River flowing into the Baltic" .)