They secure 150 snakes (6)
I believe the answer is:
clasps
'they secure' is the definition.
(clasps secure something in place)
'150 snakes' is the wordplay.
'150' becomes 'CL' (Roman numerals).
'snakes' becomes 'asps' (asp is a kind of snake).
'cl'+'asps'='CLASPS'
(Other definitions for clasps that I've seen before include "Holds the fastenings to one's bosom" , "Embraces; fastenings" , "Fasteners, as on necklaces perhaps" , "Holds tightly" , "Embraces, hugs" .)