Lance corporal ultimately introduced by dog tag (5)
I believe the answer is:
label
'tag' is the definition.
(synonyms)
'lance corporal ultimately introduced by dog' is the wordplay.
'ultimately' indicates one should take the final letters.
'introduced by' says to put letters next to each other.
'dog' becomes 'lab' (short for labrador).
The final letters of 'lance corporal' are 'el'.
'el' put after 'lab' is 'LABEL'.
(Other definitions for label that I've seen before include "Give name to" , "Name-tag" , "Sticker" , "Means of identifying" , "Tag, tab" .)