Old soldier never at assembly (7)
I believe the answer is:
veteran
'old soldier' is the definition.
(veteran can mean a former soldier)
'never at assembly' is the wordplay.
'assembly' is an anagram indicator.
'never'+'at'='neverat'
'neverat' with letters rearranged gives 'VETERAN'.
(Other definitions for veteran that I've seen before include "Former member of the armed forces" , "Warhorse" , "Long-serving" , "Old soldier's" , "Experienced ex-serviceman" .)