Mark allowed that high temperature could indicate infectious disease (7,5)
I believe the answer is:
scarlet fever
'infectious disease' is the definition.
(I've seen this before)
'mark allowed that high temperature' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'allowed' becomes 'let' (to let is to allow or permit).
'that high temperature' becomes 'fever' (I've seen this before).
'scar'+'let'+'fever'='SCARLET FEVER'
'could indicate' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for scarlet fever that I've seen before include "Infectious disease producing rash" , "Infectious disease of children" , "Disease causing rash" , "complaint" .)