Deafening silence, perhaps, after Oscar axes dunce (8)
I believe the answer is:
oxymoron
'deafening silence' is the definition.
('deafening silence' is an oxymoron)
'after oscar axes dunce' is the wordplay.
'after' means one lot of letters go next to another.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'axes' becomes 'xy' (x axis and y axis).
'dunce' becomes 'moron' (both can mean a stupid person).
'o' put next to 'xy' is 'oxy'.
'oxy'+'moron'='OXYMORON'
'perhaps' is the link.
(Other definitions for oxymoron that I've seen before include "Phrase or sentence with inherent contradiction" , "Sentence that appears to contradict itself" , "Apparently contradictory figure of speech" , "Contradiction in terms" , "'Figure of speech, it contains contradictory terms (8)'" .)