Somehow, he meant to have gas (7)
I believe the answer is:
methane
'gas' is the definition.
(I've seen this before)
'somehow he meant' is the wordplay.
'somehow' indicates an anagram.
'he'+'meant'='hemeant'
'hemeant' anagrammed gives 'METHANE'.
'to have' acts as a link.
(Other definitions for methane that I've seen before include "contributor to global warming" , "Naturally occurring flammable gas" , "simplest hydrocarbon" , "Main component of natural gas" , "Flammable gas, CH4" .)