Trendy person extremely precocious in their performing (7)
I believe the answer is:
hipster
'trendy person' is the definition.
(I've seen this before)
'extremely precocious in their performing' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'in' indicates putting letters inside.
'performing' is an anagram indicator (the letters move around or 'perform').
'precocious' with its centre removed is 'ps'.
'their' anagrammed gives 'hiter'.
'ps' put inside 'hiter' is 'HIPSTER'.
(Other definitions for hipster that I've seen before include "Dedicated follower of fashion" , "Bohemian" , "jazz he likes" , "Fashionable person" , "kind of pants" .)