Close jam factory in the end (6)
I believe the answer is:
stuffy
'close' is the definition.
(both can mean uncomfortably warm)
'jam factory in the end' is the wordplay.
'jam' becomes 'stuff' (synonyms).
'factory in the end' becomes 'y' (I am not sure about this - if you are sure you should believe this answer much more).
'stuff'+'y'='STUFFY'
(Other definitions for stuffy that I've seen before include "Airless or dull and narrow-minded" , "needing to be aired" , "Close - conventional and dull" , "Airless - humourless" , "Close, airless" .)