A girl not without heart, but peevishly impatient (8)
I believe the answer is:
petulant
'peevishly impatient' is the definition.
'petulant' can be an answer for 'impatient' (thesaurus). I am not certain of the 'peevishly' bit.
'a girl not without heart' is the wordplay.
'a girl' becomes 'petula' (I can't justify this - if you can you should believe this answer much more).
'without heart' says to hollow out the word (remove centre letters).
'not' with its middle removed is 'nt'.
'petula'+'nt'='PETULANT'
'but' is the link.
Can you help me to learn more?
(Other definitions for petulant that I've seen before include "Irritable, sullen" , "Querulous and demanding" , "Easily provoked" , "Fractious, peevish" , "Fractious, pouty" .)