S-shaped curve over letter (4)
I believe the answer is:
ogee
's-shaped curve' is the definition.
(I've seen this before)
'over letter' is the wordplay.
'over' becomes 'o' (cricket abbreviation).
'letter' becomes 'gee' (G**).
'o'+'gee'='OGEE'
(Other definitions for ogee that I've seen before include "S-shaped curve" , "architectural feature" , "S-shaped line" , "S-shaped moulding" , "bit of Gothic architecture?" .)