Like stars round a grand semicircular moulding (8)
I believe the answer is:
astragal
'semicircular moulding' is the definition.
'astragal' can be an answer for 'moulding' (astragal is a kind of moulding). I am not sure about the 'semicircular' bit.
'like stars round a grand' is the wordplay.
'like stars' becomes 'astral' (astral means relating to stars).
'round' means one lot of letters goes inside another.
'grand' becomes 'g' (G is short for grand as in a thousand).
'a'+'g'='ag'
'astral' placed around 'ag' is 'ASTRAGAL'.
Can you help me to learn more?
(Other definitions for astragal that I've seen before include "wooden strip" , "Beaded moulding for edging furniture" .)