Standard, partially declining, is never going up (6)
I believe the answer is:
ensign
'standard' is the definition.
(I've seen this before)
'partially declining is never going up' is the wordplay.
'partially' indicates a hidden word.
'going up' says the letters should be written in reverse.
'ngisne' is hidden within 'declining is never'.
'ngisne' written backwards gives 'ENSIGN'.
(Other definitions for ensign that I've seen before include "Junior US naval officer" , "Flying" , "Flag; standard-bearer" , "Soldier; standard" , "Ship's flag to show nationality" .)