To retreat is a disadvantage (8)
I believe the answer is:
drawback
'is a disadvantage' is the definition.
(I've seen this before)
'to retreat' is the wordplay.
'to' becomes 'draw' (I can't explain this - if you can you should give a lot more credence to this answer).
'retreat' becomes 'back' (to retreat is to go back).
'draw'+'back'='DRAWBACK'
Can you help me to learn more?
(Other definitions for drawback that I've seen before include "Limitation, disadvantage" , "Hindrance - disadvantage" , "Obstacle" , "Snag" , "Hindrance - retreat" .)