Escape quickly after seeing copper close to flat with ladder (3,3,3)
I believe the answer is:
cut and run
'escape quickly' is the definition.
(I've seen this before)
'seeing copper close to flat with ladder' is the wordplay.
'seeing copper' becomes 'cu' (chemical symbol).
'close to' indicates one should take the final letters (the close/ending of).
'with' becomes 'and' (I've seen this before**).
'ladder' becomes 'run' ('run' can be a synonym of 'ladder').
The final letter of 'flat' is 't'.
'cu'+'t'+'and'+'run'='CUT AND RUN'
'after' acts as a link.
(Other definitions for cut and run that I've seen before include "Make an urgent escape" , "get away quickly" , "Escape quickly from a difficult situation" , "Exit a thorny situation rather than deal with it" , "Make a rapid escape" .)