Hole up after Birkdale's seventh, one down round start of eighteenth (6)

'start of eighteenth' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'hole up after birkdale's seventh one down' is the wordplay.
'hole up' becomes 'ie' (I can't justify this - if you can you should believe this answer much more).
'after' is a charade indicator (letters next to each other).
'birkdale's seventh' becomes 'l' (7th letter of 'birkdale').
'one down' becomes 'low' (I've seen this in another clue).
'ie' put after 'l' is 'lie'.
'lie'+'low'='LIELOW'

'round' acts as a link.

Can you help me to learn more?