It turns red when nearly exhausted (5)
I believe the answer is:
tired
'nearly exhausted' is the definition.
'tired' can be an answer for 'exhausted' (exhausted can mean very tired). I'm unsure of the 'nearly' bit.
'it turns red' is the wordplay.
'turns' says the letters should be written backwards.
'it' in reverse letter order is 'ti'.
'ti'+'red'='TIRED'
'when' acts as a link.
Can you help me to learn more?
(Other definitions for tired that I've seen before include "Done in" , "Weary or fed up" , "Zonked" , "getting bored" , "Played out" .)