Energy source that’s unappealing, first off, then endlessly powerful (8)
I believe the answer is:
glycogen
'energy source' is the definition.
(substance storing energy in organisms)
'unappealing first off then endlessly powerful' is the wordplay.
'unappealing first off then' becomes 'gly' (I am not sure about this - if you are sure you should believe this answer much more).
'endlessly' means to remove the last letter.
'powerful' becomes 'cogent' (I've seen this before).
'cogent' with its last letter taken away is 'cogen'.
'gly'+'cogen'='GLYCOGEN'
'that's' acts as a link.
(Other definitions for glycogen that I've seen before include "It's stored and broken down into glucose when needed by the body" , "energy source" , "Chemical in body" .)