Successor is gas, or so we're told (4)
I believe the answer is:
heir
'successor' is the definition.
(synonyms)
'gas or so we're told' is the wordplay.
I cannot quite see how this works, but
'gas' could be 'he' (chemical symbol for helium) and 'he' is found in the answer.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for heir that I've seen before include "Son, say" , "One set to inherit" , "one's to inherit throne!" , "Expected successor" , "One who will succeed to title" .)