Examine in hospital department one with long experience in a particular field (7)
I believe the answer is:
veteran
'experience in a particular field' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'examine in hospital department one with long' is the wordplay.
I cannot really see how this works, but
'examine' could be 'vet' (I've seen this in another clue) and 'vet' is present in the answer.
'one' could be 'an' (an apple is one apple) and 'an' is found in the answer.
'in hospital' could be 'er' (I've seen this in other clues) and 'er' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for veteran that I've seen before include "Describing a pre-1905 car (or pre-1919 at a pinch?)" , "One long experienced" , "Old soldier's" , "Retired soldier" , "Long-serving" .)