Lack of two principal characters ruining scene (7)
I believe the answer is:
absence
'lack' is the definition.
(I know that absence is a type of lack)
'two principal characters ruining scene' is the wordplay.
'two principal characters' becomes 'ab' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ruining' indicates anagramming the letters.
'scene' anagrammed gives 'sence'.
'ab'+'sence'='ABSENCE'
'of' is the link.
Can you help me to learn more?
(Other definitions for absence that I've seen before include "''. . . . . . . makes the heart grow fonder'' (7)" , "State of being away" , "Lack - it makes the heart grow fonder" , "Want (of something)" , "Time off" .)