Dreadful gale with one alert (5)
I believe the answer is:
agile
'alert' is the definition.
(thesaurus)
'dreadful gale with one' is the wordplay.
'dreadful' indicates an anagram.
'with' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'gale' is an anagram of 'agle'.
'agle' enclosing 'i' is 'AGILE'.
(Other definitions for agile that I've seen before include "Limber" , "Nimble, quick-moving" , "Nimble, supple" , "Nifty" , "Flexible" .)