It may be pressed down but not up (6)
I believe the answer is:
button
'it may be pressed down' is the definition.
The definition suggests a singular noun which matches the answer.
'but not up' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'not' in reverse letter order is 'ton'.
'but'+'ton'='BUTTON'
Can you help me to learn more?
(Other definitions for button that I've seen before include "Disc for pushing, fastening" , "Push it or do it up!" , "Bright as a - -" , "Knob or disc as clothes fastener" , "Tot bun up for a fastening device" .)