Is suitable only in spasms (4)
I believe the answer is:
fits
'spasms' is the definition.
The definition and answer can be both states of affairs as well as being plural nouns.
Maybe there's a link between them I don't understand?
'is suitable only' is the wordplay.
'is suitable' becomes 'fit' ('fit' can be similar in meaning to 'suitable').
'only' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fit'+'s'='FITS'
'in' is the link.
Can you help me to learn more?
(Other definitions for fits that I've seen before include "Sudden uncontrollable attacks" , "Prepares" , "short bouts" , "Is of the right shape and size" , "feels comfortable" .)