Make little effort when in bed (5)
I believe the answer is:
coast
'make little effort' is the definition.
(to coast is work without much effort)
'when in bed' is the wordplay.
'when' becomes 'as'.
'in' means one lot of letters goes inside another.
'bed' becomes 'cot' (cot is a kind of bed).
'as' inserted within 'cot' is 'COAST'.
(Other definitions for coast that I've seen before include "Sea border" , "Land bordered by the sea" , "Move easily without power" , "Move effortlessly" , "(Of car) move without power" .)