Cross gentleman nabbing a commissioned officer (7)
I believe the answer is:
saltire
'cross' is the definition.
(saltire is a kind of cross)
'gentleman nabbing a commissioned officer' is the wordplay.
'gentleman' becomes 'sire'.
'nabbing' means one lot of letters goes inside another.
'commissioned officer' becomes 'lt' (abbreviation for lieutenant).
'a'+'lt'='alt'
'sire' placed around 'alt' is 'SALTIRE'.
(Other definitions for saltire that I've seen before include "Cross like St Andrew's" , "The Scottish flag" , "Heraldic cross" , "Scottish cross" , "Diagonal cross (heraldry)" .)