This is a charade - where two or more separate words or word parts are joined together to form the answer.
Split this into two parts: 'Distortion' gives you the first part (4 letters), and 'isn't commonly' gives you the second part (4 letters). Think about informal ways to say 'isn't'.
Answer: WARPAINT | WARP (distortion) + AIN'T (isn't commonly) = WARPAINT (cosmetic used by warriors)