Mark is cool with men on manoeuvres (9)
I believe the answer is:
semicolon
'mark' is the definition.
(semicolon is a kind of mark)
'is cool with men on manoeuvres' is the wordplay.
'with' says to put letters next to each other.
'on manoeuvres' indicates anagramming the letters (the letters manoeuvre into a new form).
'is'+'cool'='iscool'
'iscool' put after 'men' is 'meniscool'.
'meniscool' is an anagram of 'SEMICOLON'.
(Other definitions for semicolon that I've seen before include "Joiner" , "can cut sentence" , "Punctuation mark denoting a pause in a sentence" , "half a stop" , "Item of punctuation" .)