Let up, Terry! (3)
I believe the answer is:
tel
'terry' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I don't see?
'let up' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'let' reversed gives 'TEL'.
Can you help me to learn more?
(Other definitions for tel that I've seen before include "Ancient mound in Arab lands" , "Small number" , "Phone number or Terry in short" , "Phone number (abbrev.)" , "-- Aviv; old mound" .)