Foul lads go berserk in a joint abruptly (3,2,1,6)
I believe the answer is:
all of a sudden
'abruptly' is the definition.
(I've seen this before)
'foul lads go berserk in a joint' is the wordplay.
'go berserk' indicates anagramming the letters.
'in' is an insertion indicator.
'a joint' becomes 'aden' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'foul'+'lads'='foullads'
'foullads' with letters rearranged gives 'llofasud'.
'llofasud' inserted within 'aden' is 'ALL OF A SUDDEN'.
Can you help me to learn more?
(Other definitions for all of a sudden that I've seen before include "abruptly" , "Quickly and unexpectedly" , "In an unexpected manner" , "Without warning" .)