I wake up lacking energy — that's troublesome (7)
I believe the answer is:
onerous
'troublesome' is the definition.
(thesaurus)
'i wake up lacking energy' is the wordplay.
'i' becomes 'one' ('one' can be a synonym of 'i'**).
'wake up' becomes 'rouse' (I've seen this before).
'lacking' indicates named letters should be taken away.
'energy' becomes 'e' (physics symbol as in E=mcn).
'rouse' with 'e' taken away is 'rous'.
'one'+'rous'='ONEROUS'
'that's' acts as a link.
(Other definitions for onerous that I've seen before include "'Burdensome, not easily borne (7)'" , "Involving much work" , "Difficult, troublesome" , "Heavy, difficult to do or bear" , "Trying" .)