Upset father by breaking flute (7)
I believe the answer is:
fretful
'upset' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?
'father by breaking flute' is the wordplay.
'father' becomes 'fr' (abbreviation as in a priest's title).
'by' says to put letters next to each other.
'breaking' indicates anagramming the letters.
'flute' with letters rearranged gives 'etful'.
'fr'+'etful'='FRETFUL'
Can you help me to learn more?
(Other definitions for fretful that I've seen before include "Nervous and unable to relax" , "Querulous" , "Anxious" , "on edge" , "Peevish" .)