Old soldier and I might treat SEALs (3)
I believe the answer is:
vet
'old soldier' is the definition.
(veteran)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for vet that I've seen before include "Examine, check" , "Scrutinise" , "solider once" , "Examine carefully - might one help a lame duck?" , "Dog doc?" .)