Left hospital department, soldier returns with round mark on skin (7)
I believe the answer is:
lentigo
'mark on skin' is the definition.
(area of skin that's a different colour to the rest)
'left hospital department soldier returns with round' is the wordplay.
'left' becomes 'L' (common abbreviation).
'hospital department' becomes 'ENT' (abbreviation for Ear, Nose and Throat department).
'soldier' becomes 'gi' (GI can mean a US soldier).
'returns' shows that the letters should be reversed in order.
'with round' becomes 'o' (round can mean a round object. I am not sure about the 'with' bit.).
'gi' back-to-front is 'ig'.
'l'+'ent'+'ig'+'o'='LENTIGO'
(Other definitions for lentigo that I've seen before include "Spot" , "Spots" , "Skin condition" , "Skin disease" , "Technical name for a freckle" .)