Old military commander’s memory not good, sadly (10)
I believe the answer is:
montgomery
'old military commander's' is the definition.
(Field Marshal Bernard Law Montgomery)
'memory not good sadly' is the wordplay.
'good' becomes 'g' (abbreviation).
'sadly' indicates an anagram.
'memory'+'not'+'g'='memorynotg'
'memorynotg' is an anagram of 'MONTGOMERY'.
(Other definitions for montgomery that I've seen before include "Field-Marshal" , "FM?" , "capital" , "Military commander" , "Welsh division" .)