Blot out imperfect pieces, about fifty (7)
I believe the answer is:
eclipse
'blot out' is the definition.
(I've seen this before)
'imperfect pieces about fifty' is the wordplay.
'imperfect' indicates an anagram.
'about' is an insertion indicator.
'fifty' becomes 'L' (Roman numeral).
'pieces' is an anagram of 'ecipse'.
'ecipse' placed around 'l' is 'ECLIPSE'.
(Other definitions for eclipse that I've seen before include "Throw into obscurity, perhaps the sun" , "Outdo" , "dark?" , "Shadowing of sun, etc" , "Lunar or solar phenomenon" .)