Rubbish seen round top of railway cutting (4)
I believe the answer is:
tart
'cutting' is the definition.
(both can mean sharp or sarcastic)
'rubbish seen round top of railway' is the wordplay.
'rubbish' becomes 'tat' (low-quality items or material).
'seen round' means one lot of letters goes inside another.
'top of' says to take the initial letters.
The initial letter of 'railway' is 'r'.
'tat' enclosing 'r' is 'TART'.
(Other definitions for tart that I've seen before include "Sharp - bakery product" , "Pie with fruit filling" , "Bitter - sweet - floozie" , "Open cake with jam etc" , "Sharp on the tongue" .)