... but it's extremely volatile (4)
I believe the answer is:
save
'but' is the definition.
(I've seen this in other clues)
'it's extremely volatile' is the wordplay.
'it' becomes 'sa' (old-fashioned abbreviation for sex appeal).
'extremely' says to hollow out the word (remove centre letters).
'volatile' with its centre taken out is 've'.
'sa'+'ve'='SAVE'
(Other definitions for save that I've seen before include "Put by in vase" , "Rescue - economise" , "deliver from evil" , "Apart from" , "Rescue for a rainy day?" .)