Memory loss means I somehow start to ad-lib (7)
I believe the answer is:
amnesia
'memory loss' is the definition.
(synonyms)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for amnesia that I've seen before include "Inability to remember" , "Memory-loss" , "Complaint" , "Inability to recall" , "As a mine (anag)" .)