Hardy rewrote book, 'A Source of Water (7)
I believe the answer is:
hydrant
'water' is the definition.
(thesaurus)
'hardy rewrote book' is the wordplay.
'rewrote' indicates an anagram (I've seen 'rewritten' mean this).
'book' becomes 'NT' (New Testament).
'hardy' anagrammed gives 'hydra'.
'hydra'+'nt'='HYDRANT'
''a source of' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for hydrant that I've seen before include "Water supply for firefighters" , "Firemen's water supply" , "Water tap for drawing from the mains" , "Fire hose supplier" , "Water-pipe on the street for the fire brigade" .)