Passion! She's undressed with naked mate (4)
I believe the answer is:
heat
'passion' is the definition.
(I know that passion can be written as heat)
'she's undressed with naked mate' is the wordplay.
'undressed' indicates the central letters (the word removes the letters which clothe it).
'with' means one lot of letters go next to another.
'naked' means to look at the middle letters.
The centre of 'shes' is 'he'.
The middle letters of 'mate' are 'at'.
'he'+'at'='HEAT'
(Other definitions for heat that I've seen before include "Preliminary round in a contest" , "Provide warmth" , "Hotness" , "Warm up" , "High temperature" .)