Medium parts fine at Nasa, which is crazy (4)
I believe the answer is:
amok
'is crazy' is the definition.
(I've seen this in another clue)
'medium parts fine at nasa' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'parts' means one lot of letters goes inside another (in the sense of cuts or separates).
'fine at nasa' becomes 'a-ok' (I've seen this in another clue).
'm' placed within 'aok' is 'AMOK'.
'which' acts as a link.
(Other definitions for amok that I've seen before include "going wild" , "crazy" , "Going mad" , "(Run) in murderous frenzy" , "(Run) wild, out of control" .)