NCO losing Post Office pen (6)
I believe the answer is:
corral
'pen' is the definition.
(corral is a kind of pen)
'nco losing post office' is the wordplay.
'nco' becomes 'corporal' (I've seen this before).
'losing' is a deletion indicator.
'post office' becomes 'po'.
'corporal' with 'po' taken away is 'CORRAL'.
(Other definitions for corral that I've seen before include "Western horse enclosure" , "Round up" , "Livestock enclosure" , "Famous gunfight at the OK ..... in American West" , "Horse enclosure out West" .)