Military commander once more not popular (3)
I believe the answer is:
aga
'military commander' is the definition.
(I've seen this before)
'once more not popular' is the wordplay.
'once more' becomes 'again' ('again' can be a synonym of 'once more').
'not' indicates named letters should be taken away.
'popular' becomes 'in'.
'again' with 'in' taken away is 'AGA'.
(Other definitions for aga that I've seen before include "Palindromic Muslim title, as for Khan" , "Cooking range" , "Turkish ruler" , "Turkish governor" , "Eastern potentate" .)