Diminutive Sergeant-Major coming to barracks (2,5)
I believe the answer is:
in small
'barracks' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'diminutive sergeant-major coming to' is the wordplay.
'diminutive' becomes 'small' (similar in meaning).
'sergeant major' becomes 'in' (I can't explain this - if you can you should believe this answer much more).
'coming to' says to put letters next to each other.
'small' after 'in' is 'IN SMALL'.
Can you help me to learn more?
(Another definition for in small that I've seen is " Like a little model".)