Stone that covers a stick (4,2)
I believe the answer is:
stay on
'stick' is the definition.
(I've seen this before)
'stone that covers a' is the wordplay.
'stone' becomes 'st'.
'that' becomes 'yon' (archaic term meaning 'that').
'covers' means one lot of letters goes inside another.
'st'+'yon'='styon'
'styon' enclosing 'a' is 'STAY ON'.
(Other definitions for stay on that I've seen before include "Remain after others have left" , "Remain after normal time" , "stick" , "Remain behind" .)