Slip over in style (4)
I believe the answer is:
boob
'slip' is the definition.
(I've seen this before)
'over in style' is the wordplay.
'over' becomes 'o' (cricket abbreviation).
'in' means one lot of letters goes inside another.
'style' becomes 'bob' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'o' put into 'bob' is 'BOOB'.
Can you help me to learn more?
(Other definitions for boob that I've seen before include "Mistake; breast (sl.)" , "Error" , "Slip" , "Embarrassing mistake" , "Blunder (colloq.)" .)