Augmentation, initially, in the capacity of water (4)
I believe the answer is:
aqua
'water' is the definition.
(I know that Aqua is a bottled water)
'augmentation initially in the capacity of' is the wordplay.
'initially' suggests taking the first letters.
'in the capacity of' becomes 'qua' (I've seen this before).
The initial letter of 'augmentation' is 'a'.
'a'+'qua'='AQUA'
(Other definitions for aqua that I've seen before include "Water (L)" , "Light bluish-green -- water" , "Prefix denoting "water"" , "- - Libra" , "Tint of blue" .)