Crazy fool keeps on grabbing son (7)
I believe the answer is:
berserk
'crazy' is the definition.
(I've seen this before)
'fool keeps on grabbing son' is the wordplay.
'fool' becomes 'berk' (I am not sure about this - if you are sure you should believe this answer much more).
'keeps' indicates putting letters inside (inserted letters are kept inside).
'on' becomes 're' ('on' can be mean 'with reference to').
'grabbing' means one lot of letters goes inside another.
'son' becomes 's' (genealogical abbreviation for son).
're' enclosing 's' is 'rse'.
'berk' enclosing 'rse' is 'BERSERK'.
Can you help me to learn more?
(Other definitions for berserk that I've seen before include "'Frenzied, as if possessed by a demon (7)'" , "In a violent frenzy" , "Violently frenzied" , "Mad" , "Crazy" .)