Boulder in the centre blocking rise of the sun (5)
I believe the answer is:
solar
'of the sun' is the definition.
(solar means relating to the sun)
'boulder in the centre blocking rise' is the wordplay.
'boulder in the centre' becomes 'l' (I can't explain this - if you can you should give a lot more credence to this answer).
'blocking' is an insertion indicator (to block can mean to plug a gap in something).
'rise' becomes 'soar' (soaring is a kind of rising).
'l' placed into 'soar' is 'SOLAR'.
Can you help me to learn more?
(Other definitions for solar that I've seen before include "Relating to our star" , "Concerning the sun" , "Powered by the sun" , "Sort of power" , "Mediaeval chamber" .)