Succeeded, commander in long story (4)
I believe the answer is:
saga
'long story' is the definition.
(I've seen this before)
'succeeded commander' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'commander' becomes 'aga' (I've seen this before**).
's'+'aga'='SAGA'
'in' acts as a link.
(Other definitions for saga that I've seen before include "Soap opera" , "long series of events" , "Long detailed story" , "Norse narrative" , "Long and involved story" .)