Being heavy on fuel can be ominous! (7)
I believe the answer is:
fateful
'ominous' is the definition.
(I've seen this before)
'being heavy on fuel can be' is the wordplay.
'being heavy' becomes 'fat' ('fat' can be similar in meaning to 'heavy'**).
'on' is an insertion indicator.
'can be' indicates an anagram.
'fuel' is an anagram of 'eful'.
'fat' inserted into 'eful' is 'FATEFUL'.
(Other definitions for fateful that I've seen before include "Decisive" , "Portentous; bringing death" , "Ominous" , "Prophetic - portentous" , "Dire in its implications" .)