Long snake has temper (6)
I believe the answer is:
aspire
'long' is the definition.
(thesaurus)
'snake has temper' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'has' is a charade indicator (letters next to each other).
'temper' becomes 'ire' (I've seen this before).
'asp'+'ire'='ASPIRE'
(Other definitions for aspire that I've seen before include "Hope (to do something)" , "Aim (for higher things)" , "Have a strong desire to reach an objective" , "Have ambitious plans for" , "Aim, have an ambitious plan" .)