Have a siesta, keeping most comfortable (7)
I believe the answer is:
easiest
'most comfortable' is the definition.
('easy' can be a synonym of 'comfortable')
'have a siesta keeping' is the wordplay.
'keeping' indicates the answer is hidden within the clue.
'EASIEST' can be found hidden inside 'have a siesta'.
(Other definitions for easiest that I've seen before include "Least hard" , "most like pie" , "Least difficult" , "Simplest" , "most relaxed" .)