Fresh source of water first for annual purification? (6,5)
I believe the answer is:
Here is my best explanation:
'annual purification?' is the definition.
'fresh source of water first' is the wordplay.
'fresh' becomes 'clean' (I've seen this before).
'source of water' becomes 'spring' (I've seen this before).
'first' is a charade indicator (letters next to each other).
'clean' put after 'spring' is 'SPRING-CLEAN'.
'for' acts as a link.
(Other definitions for spring clean that I've seen before include "daily might do this", "Annual chore to be done", "major domestic job", "seasonal job" and "Tidy up".)