Such an officer is no major (5)
I believe the answer is:
petty
'such an officer' is the definition.
'petty' can be an answer for 'officer' (petty officer is a kind of officer). I'm unsure of the 'such an' bit.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for petty that I've seen before include "Marked by narrowness of mind" , "Trifling" , "Unimportant" , "Insignificant or trivial" , "This cash is of a trivial amount" .)