Upset commander allowed to imprison first corporal for carelessness (7)
I believe the answer is:
neglect
'carelessness' is the definition.
(neglect is a kind of carelessness)
'upset commander allowed to imprison first corporal' is the wordplay.
'upset' shows that the letters should be reversed in order.
'commander' becomes 'gen' (abbreviation for general).
'allowed to' becomes 'let' (to let something happen is to allow it).
'imprison' means one lot of letters goes inside another.
'first corporal' becomes 'c' (1st letter of 'corporal').
'gen' reversed gives 'neg'.
'let' going around 'c' is 'lect'.
'neg'+'lect'='NEGLECT'
'for' is the link.
(Other definitions for neglect that I've seen before include "Failure to give proper care" , "Disuse" , "Failure to show care" , "Lack of proper care" , "Fail to pay due attention to (7)" .)