Fairy's sea snail is sort of light blue (10)
I believe the answer is:
periwinkle
'sort of light blue' is the definition.
(shade of blue)
'fairy's sea snail' is the wordplay.
'fairy' becomes 'peri' (Persian mythological being).
'sea snail' becomes 'winkle' (type of sea mollusc).
'peri'+'winkle'='PERIWINKLE'
'is' acts as a link.
(Other definitions for periwinkle that I've seen before include "Tiny seafood item - purplish-blue shade" , "Plant with five-petalled flowers" , "Small marine snail" , "Blue" , "This plant may be poisonous" .)