Casual security ends around left (6)
I believe the answer is:
sporty
'casual' is the definition.
(thesaurus)
'security ends around left' is the wordplay.
'ends' says to hollow out the word (remove centre letters).
'around' indicates putting letters inside.
'left' becomes 'port' (left side of a ship - opposite of starboard**).
'security' with its middle taken out is 'sy'.
'sy' going around 'port' is 'SPORTY'.
(Other definitions for sporty that I've seen before include "Like some cars" , "Energetic" , "Athletic" , "(Of car) with rapid acceleration" , "Given to physical game-playing" .)