Army section leads in attack, not last usually in retreat (4)
I believe the answer is:
ulna
'army section' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'leads in attack not last usually in retreat' is the wordplay.
'leads in' says to take the initial letters (I've seen 'lead in' mean this).
'in retreat' says the letters should be written backwards.
The initial letters of 'attack not last usually' is 'anlu'.
'anlu' back-to-front is 'ULNA'.
Can you help me to learn more?
(Other definitions for ulna that I've seen before include "structural member?" , "Inner bone of the human forearm" , "Car document; ticket book" , "One in arms" , "Part of body" .)