Cunningly we reinstall a source of water (8,4)
I believe the answer is:
artesian well
'source of water' is the definition.
(I've seen this before)
'cunningly we reinstall a' is the wordplay.
'cunningly' is an anagram indicator (a cunning or tricky form of the letters).
'we'+'reinstall'+'a'='wereinstalla'
'wereinstalla' is an anagram of 'ARTESIAN WELL'.
(Other definitions for artesian well that I've seen before include "Bored source of water" , "supply of water" , "Source of water with natural pressure" , "device for getting water" .)