Excuse to knock up a Spaniard (6)
I believe the answer is:
pardon
'excuse' is the definition.
(I know that excuse can be written as pardon)
'knock up a spaniard' is the wordplay.
'knock' becomes 'rap' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'a spaniard' becomes 'don'.
'rap' back-to-front is 'par'.
'par'+'don'='PARDON'
'to' acts as a link.
(Other definitions for pardon that I've seen before include "Excuse me, kindly overlook" , "Polite expression of regret, please excuse" , "indulgence" , "Ask this to seek forgiveness" , "Forgive - eh?" .)