Truly mixed-up, mother and father (4,3,5)
I believe the answer is:
from the heart
'truly' is the definition.
Both the definition and answer are adverbs. Perhaps they are linked in a way I don't understand?
'mixed-up mother and father' is the wordplay.
'mixed up' indicates anagramming the letters.
'and' is a charade indicator (letters next to each other).
'mother' after 'father' is 'fathermother'.
'fathermother' anagrammed gives 'FROM THE HEART'.
Can you help me to learn more?
(Other definitions for from the heart that I've seen before include "Whence blood may be flowing" , "with depth of feeling" .)