One isn't properly treated for strain (7)
I believe the answer is:
tension
'strain' is the definition.
(I've seen this before)
'one isn't properly treated' is the wordplay.
'properly treated' indicates anagramming the letters.
'one'+'isnt'='oneisnt'
'oneisnt' is an anagram of 'TENSION'.
'for' is the link.
(Other definitions for tension that I've seen before include "State of strain or suspense" , "Stretched state" , "Conflict" , "One isn't (anag.)" , "Nervous strain" .)