Time in bed? A lot! (5)
I believe the answer is:
stack
'a lot' is the definition.
('stack' can be a synonym of 'lot')
'time in bed?' is the wordplay.
'time' becomes 't'.
'in' indicates putting letters inside.
'bed?' becomes 'sack' (e.g. 'in the sack'**).
't' placed into 'sack' is 'STACK'.
(Other definitions for stack that I've seen before include "A lot" , "Aggregation or heap" , "functioned successfully in Scotland" , "Piled heap" , "Place in ordered pile" .)