Show agreement and knock up warrant of absolution (6)
I believe the answer is:
pardon
'warrant of absolution' is the definition.
'pardon' can be an answer for 'absolution' (thesaurus). I'm not certain of the 'warrant of' bit.
'show agreement and knock up' is the wordplay.
'show agreement' becomes 'nod' (I've seen this in another clue).
'and' is a charade indicator (letters next to each other).
'knock' becomes 'rap' (synonyms).
'up' is a reversal indicator (in down clue: letters go upwards).
'nod'+'rap'='nodrap'
'nodrap' back-to-front is 'PARDON'.
Can you help me to learn more?
(Other definitions for pardon that I've seen before include "Forgive the polite interruption" , "Forgive (an error)" , "Forgive; come again?" , "indulgence" , "Forgive, perhaps judicially" .)