Circle sector contains gas (5)
I believe the answer is:
ozone
'gas' is the definition.
(I know that ozone is a type of gas)
'circle sector' is the wordplay.
'circle' becomes 'O' (O is a circle).
'sector' becomes 'zone' (I've seen this before).
'o'+'zone'='OZONE'
'contains' acts as a link.
(Other definitions for ozone that I've seen before include "That precious layer" , "A gas; threatened layer in air" , "bracing atmosphere" , "Bracing air (colloq.)" , "Layer that protects against ultraviolet rays" .)