Incomplete design on metal in blue (7)
I believe the answer is:
sketchy
'incomplete' is the definition.
('sketchy' can be similar in meaning to 'incomplete')
'design on metal in blue' is the wordplay.
'design on metal' becomes 'etch' (I've seen this before).
'in' means one lot of letters goes inside another.
'blue' becomes 'sky' (I've seen this in other clues).
'etch' put inside 'sky' is 'SKETCHY'.
(Other definitions for sketchy that I've seen before include "lacking detail" , "Imperfect" , "Superficial" , "Incomplete; rough" , "Imprecise" .)