A main form of obsession (5)
I believe the answer is:
mania
'obsession' is the definition.
(thesaurus)
'a main form' is the wordplay.
'form' indicates an anagram.
'a'+'main'='amain'
'amain' with letters rearranged gives 'MANIA'.
'of' is the link.
(Other definitions for mania that I've seen before include "thing" , "A (violent) mood disorder" , "serious obsession" , "Excessive enthusiasm" , "fixation" .)