Capsize act over a marine shellback (4,6)
I believe the answer is:
turn turtle
'capsize' is the definition.
(I've seen this before)
'act over a marine shellback' is the wordplay.
'act' becomes 'turn' (theatre).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'a marine shellback' becomes 'turtle' (I can't justify this - if you can you should give a lot more credence to this answer).
'turn'+'turtle'='TURN TURTLE'
Can you help me to learn more?
(Other definitions for turn turtle that I've seen before include "Tip over" , "Upend" , "Go over" , "Capsize" , "(Of boat) invert" .)