Defend former partner left in drag (7)
I believe the answer is:
explain
'defend' is the definition.
(I've seen this before)
'former partner left in drag' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'left in drag' becomes 'plain' (I am not sure about this - if you are sure you should believe this answer much more).
'ex'+'plain'='EXPLAIN'
Can you help me to learn more?
(Other definitions for explain that I've seen before include "Make understandable" , "Give reason for" , "Interpret" , "Resolve" , "Make (something) clear" .)