Bad oilman's problem is related to his belly (9)
I believe the answer is:
abdominal
'related to his belly' is the definition.
The definition suggests a singular noun but the answer is not.
'bad oilman's problem' is the wordplay.
'problem' indicates an anagram.
'bad'+'oilman'='badoilman'
'badoilman' is an anagram of 'ABDOMINAL'.
'is' is the link.
Can you help me to learn more?
(Other definitions for abdominal that I've seen before include "like tummy muscles" , "from a corporation?" , "Stomachic" , "of part of body" , "Gutsy" .)