Five sat around mammoth (4)
I believe the answer is:
vast
'mammoth' is the definition.
(thesaurus)
'five sat around' is the wordplay.
'five' becomes 'v' (roman numeral**).
'around' indicates an anagram.
'sat' anagrammed gives 'ast'.
'v'+'ast'='VAST'
(Other definitions for vast that I've seen before include "Huge, enormous" , "Astronomical" , "Gigantic" , "Colossal" , "Boundless" .)