Gas round the area (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'round the area' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'the area' becomes 'zone' (zone is a kind of area**).
'o'+'zone'='OZONE'
(Other definitions for ozone that I've seen before include "Gas; bracing sea air" , "Colourless gas, O3" , "Layer that protects against ultraviolet rays" , "Kind of oxygen layer in atmosphere, has hole in it" , "Gas that screens ultraviolet radiation" .)