Worry about gnats? (5)
I believe the answer is:
angst
'worry' is the definition.
(I've seen this before)
'about gnats?' is the wordplay.
'about' is an anagram indicator.
'gnats' anagrammed gives 'ANGST'.
(Other definitions for angst that I've seen before include "feeling worried?" , "Terrible butterflies" , "Acute sense of remorse" , "Nagging worry" , "Nonspecific sense of anxiety" .)