Guard the foolish girl (8)
I believe the answer is:
daughter
'girl' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'guard the foolish' is the wordplay.
'foolish' indicates an anagram (a foolish spelling).
'guard'+'the'='guardthe'
'guardthe' is an anagram of 'DAUGHTER'.
Can you help me to learn more?
(Other definitions for daughter that I've seen before include "Female offspring" , "Close relative" , "Female descendant" , "Family member" , "'''Don't put your ... on the stage, Mrs Worthington'' (Noel Coward) (8)'" .)