Register sick following onset of temperature (4)
I believe the answer is:
till
'register' is the definition.
(thesaurus)
'sick following onset of temperature' is the wordplay.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'following' means one lot of letters go next to another.
'onset of' suggests taking the first letters.
The initial letter of 'temperature' is 't'.
'ill' put after 't' is 'TILL'.
(Other definitions for till that I've seen before include "Work as farmer" , "Plough" , "Cultivate - where the money goes" , "Drawer holding money in shop." , "cash container" .)