Deteriorate, lose vigour (3,2,4)
I believe the answer is:
run to seed
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">deteriorate</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">lose vigour</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(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?" .)