Let up, Terry! (3)
I believe the answer is:
tel
'terry' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?
'let up' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'let' backwards is '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" .)