Ecru? Dreadfully thin colour (8)
I believe the answer is:
cerulean
'colour' is the definition.
(cerulean is a kind of colour)
'ecru? dreadfully thin' is the wordplay.
'dreadfully' indicates anagramming the letters (a dreadful spelling).
'thin' becomes 'lean' (associated in meaning).
'ecru' with letters rearranged gives 'ceru'.
'ceru'+'lean'='CERULEAN'
(Other definitions for cerulean that I've seen before include "of clear heavenly appearance" , "Deep blue colour" , "Azure" , "Sky-blue for Laurence" , "Blue like the sky" .)