Severer form of restraint (7)
I believe the answer is:
reserve
'restraint' is the definition.
(thesaurus)
'severer form' is the wordplay.
'form' indicates anagramming the letters (another form of the letters).
'severer' is an anagram of 'RESERVE'.
'of' acts as a link.
(Other definitions for reserve that I've seen before include "Save" , "Modesty- store" , "some distance" , "put to one side" , "keep for oneself" .)