Guard the foolish girl (8)
I believe the answer is:
daughter
'girl' is the definition.
(I know that girl can be written as daughter)
'guard the foolish' is the wordplay.
'foolish' is an anagram indicator (a foolish spelling).
'guard'+'the'='guardthe'
'guardthe' is an anagram of 'DAUGHTER'.
(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)'" .)