Sun god's head surrounded by halo every so often (6)
I believe the answer is:
apollo
'sun god's' is the definition.
(I've seen this in another clue)
'head surrounded by halo every so often' is the wordplay.
'head' becomes 'poll' (dialect term for the head).
'surrounded by' is an insertion indicator.
'every so often' indicates alternate letters.
The alternating letters of 'halo' are 'ao'.
'poll' inserted within 'ao' is 'APOLLO'.
(Other definitions for apollo that I've seen before include "US space program" , "US moon landing programme" , "Greek god of light, son of Zeus" , "God of light and US space programme" , "US space flight programme, 1961-72" .)