Sort of water supply perhaps not encountered before near delta (9)

'sort of water supply perhaps' is the definition.
(as in an unmetered water supply)

'not encountered before near delta' is the wordplay.
'not encountered' becomes 'unmet' ().
'before' becomes 'ere' (usually literary).
'near' says to put letters next to each other.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'unmet'+'ere'+'d'='UNMETERED'