Sleepy mum in bed - very big, with little energy (8)
I believe the answer is:
comatose
'sleepy' is the definition.
(thesaurus)
'mum in bed very big with little energy' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'in' is an insertion indicator.
'bed' becomes 'cot' (cot is a type of bed).
'very big with little energy' becomes 'ose' (I can't justify this - if you can you should believe this answer much more).
'ma' going within 'cot' is 'comat'.
'comat'+'ose'='COMATOSE'
Can you help me to learn more?
(Other definitions for comatose that I've seen before include "Come to as in drowsy condition" , "Deeply unconscious" , "In a drowsy or semi-conscious state" , "Fast asleep" , "Very drowsy, perhaps unconscious" .)