Stern leader of regiment entering mess (5)
I believe the answer is:
harsh
'stern' is the definition.
(thesaurus)
'leader of regiment entering mess' is the wordplay.
'leader of' indicates taking the first letters.
'entering' indicates putting letters inside.
'mess' becomes 'hash' (I've seen this before).
The initial letter of 'regiment' is 'r'.
'r' going inside 'hash' is 'HARSH'.
(Other definitions for harsh that I've seen before include "Unpleasantly rough" , "Severe; bleak" , "Rough and disagreeable, maybe of voice" , "Jarring" , "Unkind or cruel" .)