Mixing up names? Best to counter loss of memory (7)
I believe the answer is:
amnesia
'loss of memory' is the definition.
(I've seen this before)
'mixing up names? best to counter' is the wordplay.
'mixing up' indicates anagramming the letters.
'best' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'to counter' shows that the letters should be reversed in order.
'ai' in reverse letter order is 'ia'.
'names' with letters rearranged gives 'amnes'.
'amnes'+'ia'='AMNESIA'
(Other definitions for amnesia that I've seen before include "Mental condition" , "Main sea (anag.)" , "Severe forgetfulness" , "Clinical state of forgetfulness" , "As a mine (anag)" .)