Turn over hat dimensions (7)
I believe the answer is:
capsize
'turn over' is the definition.
(capsizing is a kind of turning over)
'hat dimensions' is the wordplay.
'hat' becomes 'cap' (I've seen this before**).
'dimensions' becomes 'size' (I've seen this before).
'cap'+'size'='CAPSIZE'
(Other definitions for capsize that I've seen before include "Upset" , "Turn so that keel faces upwards" , "Of boat, turn turtle" , "Overturn (a boat)" .)