Do not include main problem - ruling (8)
I believe the answer is:
dominant
'ruling' is the definition.
(I've seen this before)
'do not include main problem' is the wordplay.
'not' becomes 'nt' (synonyms).
'include' indicates putting letters inside.
'problem' indicates anagramming the letters.
'do'+'nt'='dont'
'main' with letters rearranged gives 'mina'.
'dont' placed around 'mina' is 'DOMINANT'.
(Other definitions for dominant that I've seen before include "Assertive" , "a key" , "Having power and influence over others" , "Soh in scale" , "Prevailing or in a commanding position" .)