Waste energy on coat I took in (8)
I believe the answer is:
emaciate
'waste' is the definition.
('emaciate' can be a synonym of 'waste')
'energy on coat i took in' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mca).
'on' says to put letters next to each other.
'coat' becomes 'mac' (mac is a kind of coat**).
'took in' becomes 'ate' ('eat' can be a synonym of 'take in').
'e'+'mac'+'i'+'ate'='EMACIATE'
(Other definitions for emaciate that I've seen before include "Waste away" , "get very thin" , "Make extremely thin" .)