Change an officer at mid-term (5)
I believe the answer is:
alter
'change' is the definition.
(I know that change can be written as alter)
'an officer at mid-term' is the wordplay.
'an' becomes 'a'.
'officer' becomes 'lt' (abbreviation for lieutenant).
'at' is a charade indicator (letters next to each other).
'mid' indicates the central letters (mid- can mean the middle of something).
The central letters of 'term' are 'er'.
'a'+'lt'+'er'='ALTER'
(Other definitions for alter that I've seen before include "Amend" , "Change or adjust" , "Bout of extravagant spending" , "reform" , "Change, maybe later" .)