Being flameproof, vests ultimately a hit with US hospital department (10)
I believe the answer is:
salamander
'being flameproof' is the definition.
(salamanders are flameproof mythical beings)
'vests ultimately a hit with us hospital department' is the wordplay.
'ultimately' suggests the final letters.
'hit' becomes 'lam' (to 'lam' means to hit hard).
'with' becomes 'and'.
'us hospital department' becomes 'ER' (emergency room).
The last letter of 'vests' is 's'.
's'+'a'+'lam'+'and'+'er'='SALAMANDER'
(Other definitions for salamander that I've seen before include "Lizard-like creature" , "Long-tailed amphibian" , "Tailed amphibian" , "Newt-like amphibian" , "Mythical fire lizard" .)