Foolish redhead and a sort of blonde (4)
I believe the answer is:
rash
'foolish' is the definition.
(thesaurus)
'redhead and a sort of blonde' is the wordplay.
'redhead' becomes 'r' (head letter of 'red').
'and' says to put letters next to each other.
'a sort of blonde' becomes 'ash' (ash blonde is a hair colour).
'r'+'ash'='RASH'
(Other definitions for rash that I've seen before include "Foolhardy with skin ailment" , "Careless" , "spate" , "Eruption on skin, imprudent" , "Over-hasty, reckless" .)