Pale blue colour (4)
I believe the answer is:
aqua
'pale blue colour' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for aqua that I've seen before include "Prefix denoting "water"" , "Tint of blue" , "Hue" , "Water; pale blue colour" , "Light bluish-green -- water" .)