Mark goes ahead when an officer leads (5)
I believe the answer is:
colon
'mark' is the definition.
(colon is a kind of mark)
'when an officer leads' is the wordplay.
'when' becomes 'on' (I've seen this in other clues).
'an officer' becomes 'col' (abbreviation for colonel, an army officer).
'leads' is a charade indicator (letters next to each other) (some letters go in front of others).
'on' put after 'col' is 'COLON'.
'goes ahead' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for colon that I've seen before include "Punctuation mark; gut" , "unit of foreign currency" , "Panama port; part of intestine" , "Stop - part of intestine" , "that may cut sentence" .)