Almost having a fit (5)
I believe the answer is:
about
'almost' is the definition.
(thesaurus)
'a fit' is the wordplay.
'fit' becomes 'bout' (I've seen this in other clues).
'a'+'bout'='ABOUT'
'having' is the link.
(Other definitions for about that I've seen before include "in the offing?" , "In conection with, all around" , "In the area of or in relation to" , "Approximately, around" , "Concerning; approximately" .)