Turn over decisive factor in what’s fitting for head? (7)
I believe the answer is:
capsize
'turn over' is the definition.
(I know that capsize is a more specific form of the action the turn over)
'fitting for head?' is the wordplay.
'fitting' becomes 'size' (I can't justify this - if you can you should give a lot more credence to this answer).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'head?' becomes 'cap' (both can mean the top of something).
'size' put after 'cap' is 'CAPSIZE'.
'decisive factor in what's' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for capsize that I've seen before include "Upset" , "Turn so that keel faces upwards" , "Overturn (a boat)" , "Of boat, turn turtle" .)