Where servicemen eat must take some cleaning up! (4)
I believe the answer is:
mess
'where servicemen eat' is the definition.
(I've seen this before)
'must take some cleaning up' is the wordplay.
'must' becomes 'mss' (I am not sure about this - if you are sure you should believe this answer much more).
'take' is an insertion indicator.
'some' says to take the centre.
'cleaning up' becomes 'neatening' ('neaten' can be a synonym of 'clean up').
The central letter of 'neatening' is 'e'.
'mss' enclosing 'e' is 'MESS'.
Can you help me to learn more?
(Other definitions for mess that I've seen before include "Untidy muddle" , "Meal" , "difficulty" , "terrible state" , "Disagreeable mixture" .)