Expiring, changing colour, losing energy (5)
I believe the answer is:
dying
'expiring' is the definition.
('die' can be a synonym of 'expire')
'changing colour losing energy' is the wordplay.
'changing colour' becomes 'dyeing' (I've seen this before).
'losing' suggests deleting specific letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'dyeing' with 'e' taken out is 'DYING'.
(Other definitions for dying that I've seen before include "Declining" , "On one's last legs" , "Coming to an end" , "Withering" , "final" .)