There is but one Joan M Clarkson birth to be found on British records between 1900 and 1920:

First name(s) JOAN M A
Last name CLARKSON
Birth year 1917
Birth quarter 2
Mother's last name Clarkson
District Bucklow
County Cheshire
Country England
Volume 8A
Page 212
Record set England & Wales Births 1837-2006
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And it looks like she was with her parents in the 1939 Register, though the birth date does not quite tie up with the above registration:

Camrose , Guildford R.D., Surrey, England

FIRST NAME(S) LAST NAME(S) DOB SEX OCCUPATION MARITAL STATUS  SCHEDULE  SCHEDULE SUB NUMBER
Joan M A Verney (Neill) 29 Jun 1918 Female Actress  Single  65  5
Evaline E Watmore 19 Jan 1882 Female Unpaid Dom Duties  Single  65  1
Edward S Clarkson ? Jul 1879 Male Architect  Married  65  2
Violet A Clarkson 11 Oct 1898 Female Unpaid Dom Duties  Married  65

The ‘Verney’ in brackets is a later change probaqbly indicating a remarriage.
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Their marriage index:

First name(s) TERENCE DE L
Last name NEILL
Marriage quarter 4
Marriage year 1938
MarriageFinder™ TERENCE DE L NEILL married
Joan M Clarkson
Spouse's last name Clarkson
District Kensington
County London
Country England
Volume 1A
Page 531
Record set England & Wales Marriages 1837-2008
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