Afternoon nap in cosiest armchair (6)
I believe the answer is:
siesta
'afternoon nap' is the definition.
(I've seen this before)
'in cosiest armchair' is the wordplay.
'in' says the answer is hidden in the clue.
'SIESTA' can be found hidden inside 'cosiest armchair'.
(Other definitions for siesta that I've seen before include "Spanish afternoon activity" , "PM snoozing?" , "Spanish afternoon nap" , "Afternoon break" , "Seat is found for a Spanish rest period" .)