Many an official who may call out youth (7)
I believe the answer is:
umpteen
'many' is the definition.
('umpteen' can be similar in meaning to 'many')
'an official who may call out youth' is the wordplay.
'an official who may call out' becomes 'ump' (abbreviation for 'umpire').
'youth' becomes 'teen' (I've seen this before).
'ump'+'teen'='UMPTEEN'
(Other definitions for umpteen that I've seen before include "Indefinitely large number" , "Indefinitely many" , "Countless" , "Ever so many" , "Unspecified large number" .)