Twenty outside church by river, it’s a very hot day (8)
I believe the answer is:
scorcher
'it's a very hot day' is the definition.
(I've seen this before)
'twenty outside church by river' is the wordplay.
'twenty' becomes 'score' (synonyms).
'outside' indicates putting letters inside.
'church' becomes 'ch' (abbreviation on maps etc).
'by' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation).
'score' placed around 'ch' is 'scorche'.
'scorche'+'r'='SCORCHER'
(Other definitions for scorcher that I've seen before include "Day of very hot and sunny weather" , "something remarkable" , "for example, really fine" , "burner" , "Informally, a really hot day - it burns!" .)