Vacating house before nuclear meltdown is very hard (9)
I believe the answer is:
herculean
'very hard' is the definition.
(as in a Herculean task)
'vacating house before nuclear meltdown' is the wordplay.
'vacating' says to hollow out the word (remove centre letters) (I've seen 'vacated' mean this (internal letters vacate the word)).
'before' says to put letters next to each other.
'meltdown' indicates an anagram (I've seen this in other clues).
'house' with its centre taken out is 'he'.
'nuclear' is an anagram of 'rculean'.
'he'+'rculean'='HERCULEAN'
'is' is the link.
(Other definitions for herculean that I've seen before include "Enormously demanding (task)" , "Colossal" , "Old Labour" , "Powerful" , "Showing great strength" .)