It may be shed? Not in onset of heat by river (8)
I believe the answer is:
outhouse
'it may be shed?' is the definition.
The definition suggests a singular noun which matches the answer.
'not in onset of heat by river' is the wordplay.
'not in' becomes 'out' ('out' is the opposite to 'in').
'onset of' suggests taking the first letters.
'by river' becomes 'ouse' (Yorkshire river. I am not sure about the 'by' bit.).
The initial letter of 'heat' is 'h'.
'out'+'h'+'ouse'='OUTHOUSE'
Can you help me to learn more?
(Other definitions for outhouse that I've seen before include "External building" , "Building near home" , "Construction" , "Smaller building separate from main one" , "Building in the grounds of a house" .)