Be quiet about temperature after exercise, seeing contagion (10)
I believe the answer is:
pestilence
'contagion' is the definition.
(both can mean infection or illness)
'be quiet about temperature after exercise' is the wordplay.
'be quiet' becomes 'silence' (both are said to tell someone to stop speaking).
'about' is an insertion indicator.
'temperature' becomes 't' (abbreviation).
'after' is a charade indicator (letters next to each other).
'exercise' becomes 'pe' (Physical Education).
'silence' enclosing 't' is 'stilence'.
'stilence' after 'pe' is 'PESTILENCE'.
'seeing' acts as a link.
(Other definitions for pestilence that I've seen before include "that'll plague you" , "Fatal epidemic disease" , "dire horseman?" , "Deadly epidemic disease" , "it's an agent of evil" .)