Old soldier in grave trouble (3)
I believe the answer is:
vet
'old soldier' is the definition.
(I've seen this before)
'in grave trouble' is the wordplay.
'in' indicates the answer is hidden within the clue.
'VET' is hidden in the letters of 'grave trouble'.
(Other definitions for vet that I've seen before include "Critically examine" , "Animal doctor, in brief" , "Old soldier (inf.)" , "animal doctor to > investigate" , "Screen" .)