Mike with ammunition tailed the huge animal once (7)
I believe the answer is:
mammoth
'huge animal once' is the definition.
(mammoths were huge animals)
'mike with ammunition tailed the' is the wordplay.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'with' is a charade indicator (letters next to each other).
'ammunition' becomes 'ammo' (synonyms).
'tailed' means to remove the last letter (to tail something can mean to remove its end).
'the' with its final letter removed is 'th'.
'm'+'ammo'+'th'='MAMMOTH'
(Other definitions for mammoth that I've seen before include "Immense" , "Huge" , "Extinct shaggy-haired elephant" , "Extinct hairy elephant" , "Gigantic - extinct animal" .)