In need of a map for The Bertie Bowl? If that's the case, I'll have to make it up! (4,6)

I believe the answer is:

lost ground

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'a map for the bertie bowl? if that's the case i'll have to make it up' is the definition.
I know nothing about this answer so I can't judge whether this works.

'in need' is the wordplay.
'in' becomes 'lost' (I am not sure about this - if you are sure you should believe this answer much more).
'need' becomes 'ground' (ground is a kind of need).
'lost'+'ground'='LOST GROUND'

'of' is the link.

Can you help me to learn more?

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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