Guy might be so loud in river (5)
I believe the answer is:
afire
'guy might be so' is the definition.
I can't tell whether this definition defines the answer.
'loud in river' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'in' means one lot of letters goes inside another.
'river' becomes 'aire' (I've seen this before).
'f' put into 'aire' is 'AFIRE'.
Can you help me to learn more?
(Other definitions for afire that I've seen before include "Ablaze" , "Burning" , "lit up" , "Alight" , "in flames" .)