Large tunnel blasted with ammo (10)
I believe the answer is:
monumental
'large' is the definition.
(similar in meaning)
'tunnel blasted with ammo' is the wordplay.
'blasted with' indicates anagramming the letters.
'tunnel' mixed up with 'ammo' gives 'MONUMENTAL'.
(Other definitions for monumental that I've seen before include "Enormous" , "Great in importance" , "Large and important" , "Stupendous" , "Massive and lasting" .)