A medical procedure, in plain language not entirely relevant (7)
I believe the answer is:
apropos
'relevant' is the definition.
(I've seen this before)
'a medical procedure in plain language not entirely' is the wordplay.
'medical procedure' becomes 'op' (short for operation).
'in' indicates putting letters inside.
'plain language' becomes 'prose' (I've seen this in other clues).
'not entirely' means to remove the last letter (most but not all of the word).
'prose' with its final letter taken away is 'pros'.
'op' going inside 'pros' is 'propos'.
'a'+'propos'='APROPOS'
(Other definitions for apropos that I've seen before include "As regards" , "On the subject of" , "suitably" , "by the way" , "Concerned with" .)