Sue is of use (7)
I believe the answer is:
anagram
This clue could be a double definition.
'sue' is the first definition.
Although both the answer and definition are verbs in their base form, I cannot understand how they can define each other.
'of use' is the second definition.
The definition suggests a singular noun which matches the answer.
'is' is the link.
Can you help me to learn more?
(Other definitions for anagram that I've seen before include "Jumbling of letters" , "Word play" , "Mix-up" , ""Diagnose" is one of "San Diego"" , "Word formed by rearranging another word" .)