American pier not quite in water (4)
I believe the answer is:
aqua
'water' is the definition.
(I know that water can be written as aqua)
'american pier not quite' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'pier' becomes 'quay' (I've seen this in another clue).
'not quite' means to remove the last letter (most but not all of the word).
'quay' with its last letter removed is 'qua'.
'a'+'qua'='AQUA'
'in' acts as a link.
(Other definitions for aqua that I've seen before include "-- fortis, 'strong water'" , "Blue tinged with green" , "Tint of blue" , "which supports boats?" , "Prefix denoting "water"" .)