Girls worried about the first additive - it's chewy (7)
I believe the answer is:
gristle
'additive it's chewy' is the definition.
I can't judge whether this defines the answer.
'girls worried about the first' is the wordplay.
'worried' is an anagram indicator (worry can mean to interfere with or disturb).
'about' means one lot of letters goes inside another.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'first' becomes 'e' (I can't explain this - if you can you should believe this answer much more).
'girls' with letters rearranged gives 'grisl'.
'grisl' going around 't' is 'gristl'.
'gristl'+'e'='GRISTLE'
Can you help me to learn more?
(Other definitions for gristle that I've seen before include "Sinew" , "Chewy cartilage" , "Cartilage in meat" , "Stir leg (anag.)" , "Tough flexible tissue" .)