Catch great snake (5)
I believe the answer is:
grasp
'catch' is the definition.
(thesaurus)
'great snake' is the wordplay.
'great' becomes 'gr' (abbreviation used in place names).
'snake' becomes 'asp' (asp is a kind of snake).
'gr'+'asp'='GRASP'
(Other definitions for grasp that I've seen before include "Understand; take in hand" , "Appreciation" , "Hold - understanding" , "Take firm hold" , "Understand; hold tight" .)