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