Yours truly when terribly rude must be restrained (8)
I believe the answer is:
measured
'restrained' is the definition.
(I've seen this before)
'yours truly when terribly rude' is the wordplay.
'yours truly' becomes 'me'.
'when' becomes 'as'.
'terribly' indicates anagramming the letters.
'rude' is an anagram of 'ured'.
'me'+'as'+'ured'='MEASURED'
'must be' is the link.
(Other definitions for measured that I've seen before include "Sober" , "Found the size or quantity of something - using tape or ruler" , "Sized; carefully chosen" , "Regular in movement or carefully considered" , "Weighed" .)