Officer wanting short afternoon nap is most comfortable (7)
I believe the answer is:
cosiest
'most comfortable' is the definition.
('cosy' can be similar in meaning to 'comfortable')
'officer wanting short afternoon nap' is the wordplay.
'officer' becomes 'co' (Commanding Officer abbreviation).
'wanting' means one lot of letters go next to another.
'short' means to remove the last letter.
'afternoon nap' becomes 'siesta' (I've seen this before).
'siesta' with its final letter taken off is 'siest'.
'co'+'siest'='COSIEST'
'is' acts as a link.
(Another definition for cosiest that I've seen is " Most warm and comfortable".)