They’re extremely keen to welcome new youngster (8)
I believe the answer is:
teenager
'youngster' is the definition.
(I know that teenager is a type of youngster)
'they're extremely keen to welcome new' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'keen' becomes 'eager' (I've seen this before).
'to welcome' indicates putting letters inside (in sense of 'inviting in').
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'theyre' with its middle removed is 'te'.
'eager' placed around 'n' is 'enager'.
'te'+'enager'='TEENAGER'
(Other definitions for teenager that I've seen before include "youth, perhaps" , "Generate (anag)" , "14, perhaps" , "one that's not yet made a score?" , "Old child or young adult?" .)