Whereupon you just need the outer parts (4)
I believe the answer is:
when
'whereupon' is the definition.
(thesaurus)
'you just need the outer parts' is the wordplay.
'you just' becomes 'wn' (I am not sure about this - if you are sure you should believe this answer much more).
'need' becomes 'have' (synonyms).
'the outer' means to remove the middle letters.
'parts' indicates putting letters inside (inserted letters separate or part others).
'have' with its centre taken out is 'he'.
'wn' placed around 'he' is 'WHEN'.
Can you help me to learn more?
(Other definitions for when that I've seen before include "At what time?" , "stop pouring the drink" , "As soon as" , "Over what period?" , "Say it when you''ve had enough" .)