Is apt to make outbursts! (4)
I believe the answer is:
fits
'outbursts' is the definition.
('fit' can be a synonym of 'outburst')
'is apt to' is the wordplay.
'is apt' becomes 'fit' (I've seen this before).
'to' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'fit'+'s'='FITS'
'make' acts as a link.
Can you help me to learn more?
(Other definitions for fits that I've seen before include "Paroxysms" , "short bouts" , "Seizures" , "crises once" , "Prepares" .)