One with several partners, mostly a pig misbehaving (10)
I believe the answer is:
polygamist
'one with several partners' is the definition.
'mostly a pig misbehaving' is the wordplay.
'misbehaving' is an anagram indicator.
'mostly'+'a'+'pig'='mostlyapig'
'mostlyapig' is an anagram of 'POLYGAMIST'.
(Other definitions for polygamist that I've seen before include "Person multiply married" , "One with multiple spouses" , "One multiply-spliced" , "Great family man" , "not being single-minded" .)