Special gas ring for pudding (4)
I believe the answer is:
sago
'pudding' is the definition.
(I've seen this before)
'special gas ring' is the wordplay.
'special' is an anagram indicator.
'ring' becomes 'o'.
'gas' is an anagram of 'sag'.
'sag'+'o'='SAGO'
'for' acts as a link.
(Other definitions for sago that I've seen before include "Starchy food as white pudding" , "Powdery starchy food" , "Powdery starch from a palm" , "Starchy foodstuff" , "Starchy grain used in desserts" .)