Drop in temperature? Going up around chimney (7)
I believe the answer is:
plummet
'drop' is the definition.
(plummeting is a kind of dropping)
'temperature? going up around chimney' is the wordplay.
'temperature?' becomes 'temp'.
'going up' says the letters should be written in reverse.
'around' is an insertion indicator.
'chimney' becomes 'lum' (I've seen this in other clues).
'temp' backwards is 'pmet'.
'pmet' placed around 'lum' is 'PLUMMET'.
'in' acts as a link.
(Other definitions for plummet that I've seen before include "Nose-dive" , "Fall rapidly" , "Drop vertically and fast" , "Drop straight down" , "Fall at high speed" .)