A mother cracking up foremost computer nut (9)
I believe the answer is:
macadamia
'foremost computer nut' is the definition.
'macadamia' can be an answer for 'nut' (type of nut). I am unsure of the 'foremost computer' bit.
'a mother cracking up' is the wordplay.
'a mother' becomes 'macadam' (I can't explain this - if you can you should believe this answer much more).
'cracking' becomes 'AI' (resembles Am, informal term for excellent).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'ai' back-to-front is 'ia'.
'macadam'+'ia'='MACADAMIA'
Can you help me to learn more?
(Other definitions for macadamia that I've seen before include "(Of drug) causing dependency" , "nut-tree" , "Edible nut" , "Am I a cad, ma? (anag) - Australian nut tree" , "Australian tree with edible nuts - a CIA madam (anag)" .)