Said yes, the old soldier had come in (6)
I believe the answer is:
agreed
'said yes' is the definition.
(I've seen this before)
'old soldier had come in' is the wordplay.
I cannot really see how this works, but
'soldier' could be 're' (Royal Engineer) and 're' is present in the answer.
'old' could be 'aged' (similar in meaning) and 'aged' is found within the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'the' is the link.
Can you help me to learn more?
(Other definitions for agreed that I've seen before include "Harmonised" , "acceptable" , "Expressed willingness" , "Consented and concurred" , "Assented, acquiesced" .)