Hatters top -- form exceptional, mainly (3,3,4,4)
I believe the answer is:
for the most part
'mainly' is the definition.
(thesaurus)
'hatters top form exceptional' is the wordplay.
'exceptional' indicates an anagram.
'hatters'+'top'+'form'='hatterstopform'
'hatterstopform' is an anagram of 'FOR THE MOST PART'.
(Other definitions for for the most part that I've seen before include "Mainly" , "in general" , "Chiefly" , "Usually" .)