Rain-cloud - I'm black blocking sun to the west (6)
I believe the answer is:
nimbus
'rain-cloud' is the definition.
(I've seen this before)
'i'm black blocking sun to the west' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'blocking' means one lot of letters goes inside another.
'to the west' shows that the letters should be reversed in order.
'sun' backwards is 'nus'.
'im'+'b'='imb'
'imb' going inside 'nus' is 'NIMBUS'.
(Other definitions for nimbus that I've seen before include "Type of rain cloud" , "round one's head?" , "A kind of rain-cloud, or 15 Across" , "Large rain cloud; halo" , "A black rain cloud" .)