That woman on medical TV show is thin (5)
I believe the answer is:
sheer
'is thin' is the definition.
(similar in meaning)
'that woman on medical tv show' is the wordplay.
'that woman' becomes 'she' (she refers to a woman).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'medical tv show' becomes 'ER' (US medical drama).
'she'+'er'='SHEER'
(Other definitions for sheer that I've seen before include "Completely" , "Outright" , "Very thin as in ladies' stockings" , "Very steep; utter" , "Very fine or vertical" .)