American gas, say, promoting energy as serviceable (6)
I believe the answer is:
useful
'serviceable' is the definition.
(associated in meaning)
'american gas say promoting energy' is the wordplay.
I cannot quite see how this works, but
'american' could be 'us' (abbreviation for United States) and 'us' is found in the answer.
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is present in the answer.
This explanation may well be incorrect...
'as' acts as a link.
(Other definitions for useful that I've seen before include "Suitable for practical purpose" , "Functional and worthwhile" , "Practical and functional" , "Handy; very able" , "Make yourself . . . . . . - help out" .)