Be unhappy about reduction in desire coming up again (9)
I believe the answer is:
resurgent
'coming up again' is the definition.
(resurgent means reviving or coming back)
'be unhappy about reduction in desire' is the wordplay.
'be unhappy' becomes 'resent' (to resent something is to be unhappy about it).
'about' indicates putting letters inside.
'reduction in' means to remove the last letter (the number of letters in the word is reduced).
'desire' becomes 'urge' (I've seen this before).
'urge' with its final letter removed is 'urg'.
'resent' enclosing 'urg' is 'RESURGENT'.
(Other definitions for resurgent that I've seen before include "given new life" , "making comeback" , "reborn" , "Coming back to life" , "Reviving after a lull" .)