Number One resentful about gradual decline (7)
I believe the answer is:
erosion
'gradual decline' is the definition.
(I've seen this before)
'number one resentful about' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'one' becomes 'i' (Roman numeral).
'resentful' becomes 'sore' (I've seen this before).
'about' says the letters should be written backwards.
'no'+'i'+'sore'='noisore'
'noisore' reversed gives 'EROSION'.
(Other definitions for erosion that I've seen before include "Abrasion" , "Gradual decline" , "Wearing away (of soil, rocks etc)" , "Process of wearing away, by water or time" , "Process of wear" .)