Sustained by limitless good tea (6)
I believe the answer is:
oolong
'tea' is the definition.
(I know that oolong is a type of tea)
'sustained by limitless good' is the wordplay.
'sustained' becomes 'long' (I've seen this before).
'by' means one lot of letters go next to another.
'limitless' indicates the central letters.
The middle of 'good' is 'oo'.
'long' put after 'oo' is 'OOLONG'.
(Other definitions for oolong that I've seen before include "oriental tea" , "sort of leaves" , "Chinese tea leaves - no logo (anag)" , "A variety of black China tea with green tea flavour" , "Kind of China tea" .)