Little terror had to catch new flu variant (7)
I believe the answer is:
handful
'little' is the definition.
(thesaurus)
'had to catch new flu variant' is the wordplay.
'to catch' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'flu variant' becomes 'ful' (I can't justify this - if you can you should believe this answer much more).
'had' placed around 'n' is 'hand'.
'hand'+'ful'='HANDFUL'
'terror' is the link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for handful that I've seen before include "Someone hard to control" , "Something hard to control - small number" , "'Small number, a fistful (7)'" , "Perhaps lively child" , "Small number or amount" .)