Oscar won by impersonator - ridiculous and irregular (9)
I believe the answer is:
aperiodic
'irregular' is the definition.
(not occurring regularly or periodically)
'oscar won by impersonator' is the wordplay.
'oscar won' becomes 'iodic' (I am not sure about this - if you are sure you should believe this answer much more).
'by' means one lot of letters go next to another.
'impersonator' becomes 'aper' (I've seen this in another clue).
'iodic' after 'aper' is 'APERIODIC'.
'ridiculous and' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for aperiodic that I've seen before include "without regular metre?" , "at irregular intervals" .)