Party left fuel out - that's sad (7)
I believe the answer is:
doleful
'sad' is the definition.
(I've seen this before)
'party left fuel out' is the wordplay.
'party' becomes 'do' (do can informally mean a party).
'left' becomes 'l' (common abbreviation).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'fuel' with letters rearranged gives 'eful'.
'do'+'l'+'eful'='DOLEFUL'
'that's' is the link.
(Other definitions for doleful that I've seen before include "Sad" , "Miserable" , "not happy?" , "Cheerless" , "down in the dumps" .)