Sweltering temperature, very unpleasant but not hot (6)
I believe the answer is:
torrid
'sweltering' is the definition.
(I've seen this in another clue)
'temperature very unpleasant but not hot' is the wordplay.
'temperature' becomes 't' (abbreviation).
'very unpleasant' becomes 'horrid' (I've seen this before).
'but not' suggests deleting specific letters.
'hot' becomes 'h' (abbreviation eg on taps).
'horrid' with 'h' taken out is 'orrid'.
't'+'orrid'='TORRID'
(Other definitions for torrid that I've seen before include "Scorching hot" , "Dried with heat or intensely passionate" , "Extremely hot" , "sultry atmosphere" , "Highly emotional" .)