Mend? Oh no (4)
I believe the answer is:
darn
'mend?' is the definition.
(I know that mend can be written as darn)
'oh no' is the wordplay.
'oh' becomes 'dar' (I can't explain this - if you can you should give a lot more credence to this answer).
'no' becomes 'n' (common abbreviation).
'dar'+'n'='DARN'
Can you help me to learn more?
(Other definitions for darn that I've seen before include "Mend with a needle" , "Mend (woollens)" , "Mend by stitching - damn!" , "Mend woollen items" , "Mend with wool using needles" .)