Copy key first — and do a runner! (6)
I believe the answer is:
escape
'do a runner' is the definition.
(I've seen this before)
'copy key first' is the wordplay.
'copy' becomes 'ape' ('ape' can be a synonym of 'copy').
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'first' says to put letters next to each other.
'ape' put after 'esc' is 'ESCAPE'.
'and' is the link.
(Other definitions for escape that I've seen before include "Slip away - leak out" , "Get away from depravity" , "Gain freedom" , "find way out" , "Abscond, flee" .)