He's not in mess hall, in short (5)
I believe the answer is:
small
'short' is the definition.
('small' is associated in meaning with 'short')
'he's not in mess hall' is the wordplay.
I cannot really see how this works, but
'he' could be 'mal' and 'mal' is found within the answer.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for small that I've seen before include "Limited in size or scope" , "Lower middle of the back" , "Itsy-bitsy" , "Brief" , "Not great (in number)" .)