If you want to plead, ring dad up first (6)
I believe the answer is:
appeal
'if you want to plead' is the definition.
I can't tell whether this defines the answer.
'ring dad up first' is the wordplay.
'ring' becomes 'peal' (pealing is a kind of ringing).
'dad' becomes 'pa' (both can mean one's father).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'first' says to put letters next to each other.
'pa' back-to-front is 'ap'.
'peal' after 'ap' is 'APPEAL'.
Can you help me to learn more?
(Other definitions for appeal that I've seen before include "Try again in higher court" , "Draw" , "fetching" , "Petition to a higher court" , "Refer sentence to higher court for review" .)