Race in streak being impatient (5)
I believe the answer is:
ratty
'impatient' is the definition.
(thesaurus)
'race in streak' is the wordplay.
'race' becomes 'tt'.
'in' indicates putting letters inside.
'streak' becomes 'ray' (as in a streak of light).
'tt' put within 'ray' is 'RATTY'.
'being' acts as a link.
(Other definitions for ratty that I've seen before include "Bad tempered" , "Moley's friend" , "Mole's friend (The Wind in the Willows)" , "infestation of vermin" , "Character created by Kenneth Grahame" .)