It’s rough when a raw beginner gets in a mess (5)
I believe the answer is:
harsh
'it's rough' is the definition.
('harsh' can be a synonym of 'rough')
'a raw beginner gets in a mess' is the wordplay.
'beginner' indicates taking the first letters.
'gets in' means one lot of letters goes inside another.
'a mess' becomes 'hash' (I've seen this before).
The initial letter of 'raw' is 'r'.
'r' going inside 'hash' is 'HARSH'.
'when' is the link.
(Other definitions for harsh that I've seen before include "Unpleasantly stern" , "Jarring" , "Severe - unkind" , "Punitive" , "Raucous - austere - bitter" .)