Long snake that is crossing river (6)
I believe the answer is:
aspire
'long' is the definition.
(thesaurus)
'snake that is crossing river' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'crossing' is an insertion indicator.
'river' becomes 'r'.
'ie' enclosing 'r' is 'ire'.
'asp'+'ire'='ASPIRE'
(Other definitions for aspire that I've seen before include "Yearn, dream (towards)" , "I spare (anag.)" , "Direct one's hopes and ambitions" , "Desire (to reach higher things)" , "Aim at high things" .)