Skin problem slowly covering soldier from behind (7)
I believe the answer is:
lentigo
'skin problem' is the definition.
(lentigo is a skin condition)
'slowly covering soldier from behind' is the wordplay.
'slowly' becomes 'lento' (synonyms).
'covering' is an insertion indicator.
'soldier' becomes 'gi' (GI can mean a US soldier).
'from behind' is a reversal indicator.
'gi' backwards is 'ig'.
'lento' placed around 'ig' is 'LENTIGO'.
(Other definitions for lentigo that I've seen before include "Skin disease" , "Spot" , "Technical name for a freckle" , "Skin condition" , "Spots" .)