American to drink deeply (but not very loud) water (4)
I believe the answer is:
aqua
'water' is the definition.
(I know that Aqua is a bottled water)
'american to drink deeply but not very loud' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'to drink deeply' becomes 'quaff' (I've seen this before).
'but not' indicates named letters should be taken away.
'very loud' becomes 'ff' (musical abbreviation).
'quaff' with 'ff' taken away is 'qua'.
'a'+'qua'='AQUA'
(Other definitions for aqua that I've seen before include "-- fortis, 'strong water'" , "shade" , "Latin for water" , "- - Libra" , "Water; pale blue colour" .)