Capsize act with the terrapin (4,6)
I believe the answer is:
turn turtle
'capsize' is the definition.
(I've seen this before)
'act with the terrapin' is the wordplay.
'act' becomes 'turn' (theatre).
'with' means one lot of letters go next to another.
'the terrapin' becomes 'turtle' (I've seen this in another clue).
'turn'+'turtle'='TURN TURTLE'
(Other definitions for turn turtle that I've seen before include "Capsize" , "Upend" , "Tip over" , "(Of boat) invert" , "Go over" .)