What disrupted boy, reading with leaderless group (1,3,4,6)
I believe the answer is:
i beg your pardon
'what' is the definition.
(I've seen this before)
'disrupted boy reading with leaderless group' is the wordplay.
'disrupted' indicates an anagram.
'with' means one lot of letters go next to another.
'leaderless' means to remove the first letter.
'group' with its first letter removed is 'roup'.
'boy'+'reading'='boyreading'
'boyreading' after 'roup' is 'roupboyreading'.
'roupboyreading' anagrammed gives 'I BEG YOUR PARDON?'.
(Other definitions for i beg your pardon that I've seen before include "come again" , "let me off, please" , "Sorry" , "Please forgive me" .)