Not a sure footed mule (7)
I believe the answer is:
slipper
'sure footed mule' is the definition.
'slipper' can be an answer for 'mule' (I've seen this in another clue). I am not sure about the 'sure footed' bit.
'not a' is the wordplay.
'not' becomes 'slip' (I can't justify this - if you can you should give a lot more credence to this answer).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'slip'+'per'='SLIPPER'
Can you help me to learn more?
(Other definitions for slipper that I've seen before include "Accident-prone" , "Casual indoors shoe" , "Indoor shoe" , "Nearby; pub" , "One losing footing" .)