Underwear -- if one has drag on, that is (8)
I believe the answer is:
lingerie
'underwear' is the definition.
(type of underwear)
'one has drag on that is' is the wordplay.
'one has drag on' becomes 'linger' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'linger'+'ie'='LINGERIE'
'if' is the link.
Can you help me to learn more?
(Other definitions for lingerie that I've seen before include "Women's flimsy underwear" , "One from W Atlantic island group" , "flimsy stuff" , "Fancy underwear for women" , "Scanties" .)