Very fast promise sits awkwardly (11)
I believe the answer is:
prestissimo
'very fast' is the definition.
'promise sits awkwardly' is the wordplay.
'awkwardly' is an anagram indicator.
'promise'+'sits'='promisesits'
'promisesits' is an anagram of 'PRESTISSIMO'.
(Other definitions for prestissimo that I've seen before include "Rapid" , "extremely quickly" , "In a very quick tempo; music" , "at great speed" , "at breakneck speed" .)