A god has a halo perhaps, surrounding head (6)
I believe the answer is:
apollo
'a god' is the definition.
(Apollo is a Greek god)
'a halo perhaps surrounding head' is the wordplay.
'a halo perhaps' becomes 'ao' (I can't justify this - if you can you should give a lot more credence to this answer).
'surrounding' is an insertion indicator.
'head' becomes 'poll' (poll is a now rare word for head or scalp).
'ao' enclosing 'poll' is 'APOLLO'.
'has' is the link.
Can you help me to learn more?
(Other definitions for apollo that I've seen before include "Handsome young man of Greek mythology" , "source of name for far-flung missions" , "Greek god of light, son of Zeus" , "Lad who's striking" , "US space flight programme, 1961-72" .)