Stuck on six, annoyingly all clustered in the same corner.
Assistance with these three might move my mind into a higher gear :
50A - Young horse last in race, behind one of eighteen (7)
T???A??
55A - Demolish a cliff, blasted in malice (11)
&
46D - Hopeful cop in decline (7)
W?K????
Some of the letters suggested may be erroneous