I mess about, remarkably sober (10)
I believe the answer is:
abstemious
'sober' is the definition.
(thesaurus)
'i mess about remarkably' is the wordplay.
'remarkably' indicates an anagram.
'i'+'mess'+'about'='imessabout'
'imessabout' is an anagram of 'ABSTEMIOUS'.
(Other definitions for abstemious that I've seen before include "in a way that's restrained" , "not eating or drinking a lot" , "Restrained in one's appetites" , "I mess about (anag.)" , "Indulging only moderately (in food/drink)" .)