Deteriorate, lose vigour (3,2,4)
I believe the answer is:
run to seed
I believe this clue is a double definition.
'deteriorate' is the first definition.
(I've seen this before)
'lose vigour' is the second definition.
(I've seen this before)
(Other definitions for run to seed that I've seen before include "exhausted" , "Get shabby" , "Degenerate" , "Tennis fan after prized autograph may" , "Deteriorate in appearance or cease flowering - on the trot?" .)