Fire is terribly widespread (4)
I believe the answer is:
rife
'widespread' is the definition.
(thesaurus)
'fire is terribly' is the wordplay.
'is terribly' is an anagram indicator.
'fire' anagrammed gives 'RIFE'.
(Other definitions for rife that I've seen before include "Of widespread occurrence" , "raging" , "Overabundant" , "Very common or frequently occurring" , "Excessively abundant" .)