Vast mines collapsing around me (7)
I believe the answer is:
immense
'vast' is the definition.
(I've seen this before)
'mines collapsing around me' is the wordplay.
'collapsing' indicates anagramming the letters.
'around' means one lot of letters goes inside another.
'mines' with letters rearranged gives 'imnse'.
'imnse' placed around 'me' is 'IMMENSE'.
(Other definitions for immense that I've seen before include "Very big, exceedingly great" , "Unctuously ingratiating" , "Extremely large or great" , "Jumbo" , "Huge, very big indeed" .)