Reaction, by the sound of it, when apartment evacuated and I go away! (7)
I believe the answer is:
atishoo
'reaction by the sound of it' is the definition.
(word representing the sound of a sneeze)
'apartment evacuated and i go away' is the wordplay.
'apartment' becomes 'apt' (abbreviation in property ads for example).
'evacuated' says to hollow out the word (remove centre letters).
'and' says to put letters next to each other.
'go away' becomes 'shoo' (I've seen this before).
'apt' with its centre taken out is 'at'.
'at'+'i'+'shoo'='ATISHOO'
'when' is the link.
(Other definitions for atishoo that I've seen before include "Sound of explosion" , "hay-fever sufferer may interject this" , "report of involuntary escape from trap?" , "Sneezing sound" , "Sudden eruption" .)