It's rough when a raw beginner gets in a mess (5)
I believe the answer is:
harsh
'it's rough' is the definition.
(synonyms)
'a raw beginner gets in a mess' is the wordplay.
'beginner' says to take the initial letters.
'gets in' indicates putting letters inside.
'a mess' becomes 'hash' (as in making a hash of something).
The first letter of 'raw' is 'r'.
'r' inserted into '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" .)