Mock endless serene composition (5)
I believe the answer is:
sneer
'mock' is the definition.
(I've seen this in another clue)
'endless serene composition' is the wordplay.
'endless' means to remove the last letter.
'composition' indicates an anagram (letters arranged or composed in a different order).
'serene' with its final letter taken off is 'seren'.
'seren' is an anagram of 'SNEER'.
(Other definitions for sneer that I've seen before include "Show contempt with scornful expression or words" , "Taunt" , "Derisive remark" , "Express cynical contempt" , "sound scornful" .)