Disposal must cover one barrel for water (8)
I believe the answer is:
salivate
'water' is the definition.
(as in your mouth watering)
'disposal must cover one barrel' is the wordplay.
'disposal' becomes 'sale' (to sell something is to dispose of it).
'must cover' indicates putting letters inside.
'one' becomes 'I' (Roman numeral).
'barrel' becomes 'vat' (I've seen this before).
'i'+'vat'='ivat'
'sale' going around 'ivat' is 'SALIVATE'.
'for' acts as a link.
(Other definitions for salivate that I've seen before include "Lisa a vet (anag.)" , "Make mouth moisture in aestival manner" , "Produce moisture in mouth" , "Become moist in mouth" , "Water at the mouth" .)