Start of evening and sloppy seat is most comfortable (7)
I believe the answer is:
easiest
'most comfortable' is the definition.
('easy' can be similar in meaning to 'comfortable')
'start of evening and sloppy seat is' is the wordplay.
'start of' suggests taking the first letters.
'and' says to put letters next to each other.
'sloppy' indicates an anagram.
The initial letter of 'evening' is 'e'.
'seat'+'is'='seatis'
'seatis' anagrammed gives 'asiest'.
'e'+'asiest'='EASIEST'
(Other definitions for easiest that I've seen before include "Requiring the least amount of effort" , "Most simple" , "Posing least difficulty" , "most relaxed" , "Simplest" .)