Bitter Roger's getting into mess (5)
I believe the answer is:
harsh
'bitter' is the definition.
(harsh can mean bitter or severe)
'roger's getting into mess' is the wordplay.
'roger's' becomes 'r's' (abbreviation used in radio communications).
'getting into' indicates putting letters inside.
'mess' becomes 'hah' (I am not sure about this - if you are sure you should believe this answer much more).
'rs' inserted inside 'hah' is 'HARSH'.
Can you help me to learn more?
(Other definitions for harsh that I've seen before include "'Severe, rough (5)'" , "Severe and disagreeable" , "Draconian" , "Jarring" , "Guttural" .)