Blue fish tank half-hidden (4)
I believe the answer is:
aqua
'blue' is the definition.
(aqua is a kind of blue)
'fish tank half-hidden' is the wordplay.
'fish tank' becomes 'aquarium' (synonyms).
'half-hidden' means to take half the letters.
'aquarium' with half the letters taken is 'AQUA'.
(Other definitions for aqua that I've seen before include "Blue tinged with green" , "shade" , "- - Libra" , "Water (L)" , "Water; pale blue colour" .)