Poisonous gas stored in frozen central unit (5)
I believe the answer is:
ozone
'poisonous gas stored' is the definition.
I can't judge whether this defines the answer.
'frozen central unit' is the wordplay.
'central' means to look at the middle letters.
'unit' becomes 'one' (synonyms).
The middle of 'frozen' is 'oz'.
'oz'+'one'='OZONE'
'in' is the link.
Can you help me to learn more?
(Other definitions for ozone that I've seen before include "Allotrope of oxygen" , "It's said to brace" , "Gas; bracing sea air" , "Sea air; toxic gas" , "Layer that protects against ultraviolet rays" .)