Old soldiers' change of vest (4)
I believe the answer is:
vets
'old soldiers'' is the definition.
(short for veterans)
'change of vest' is the wordplay.
'change of' is an anagram indicator (the letter order is changed).
'vest' anagrammed gives 'VETS'.
(Other definitions for vets that I've seen before include "Screens the animal doctors" , "Siegfried and Tristan Farnon, for example" , "Thoroughly examines" , "Checks out" , "Examines the animal doctors carefully?" .)