In court, youngster takes in many at first — very many! (7)
I believe the answer is:
umpteen
'very many' is the definition.
(I've seen this before)
'in court youngster takes in many at first' is the wordplay.
'in court' becomes 'up'.
'youngster' becomes 'teen' (I've seen this before**).
'takes in' indicates putting letters inside.
'at first' says to take the initial letters.
The initial letter of 'many' is 'm'.
'up'+'teen'='upteen'
'upteen' placed around 'm' is 'UMPTEEN'.
(Other definitions for umpteen that I've seen before include "Great in number but unspecified" , "Indefinitely many" , "An uncountable number" , "Lots of things on pet menu" , "Guarded" .)