Frantic alert has time to unnerve (6)
I believe the answer is:
rattle
'unnerve' is the definition.
(thesaurus)
'frantic alert has time' is the wordplay.
'frantic' indicates anagramming the letters (I've seen this in other clues).
'has' means one lot of letters goes inside another.
'time' becomes 't'.
'alert' anagrammed gives 'ratle'.
'ratle' enclosing 't' is 'RATTLE'.
'to' is the link.
(Other definitions for rattle that I've seen before include "Unnerve - baby's plaything" , "American viper" , "Gurgling sound in throat" , "Fluster" , "Baby's toy; irritate" .)