Happen to call about absolutely nothing (6)
I believe the answer is:
befall
'happen to' is the definition.
(synonyms)
'call about absolutely nothing' is the wordplay.
'call' becomes 'bell' (bell can mean a phone call).
'about' indicates putting letters inside.
'absolutely nothing' becomes 'FA' (slang term - sweet Fanny Adams).
'bell' enclosing 'fa' is 'BEFALL'.
(Other definitions for befall that I've seen before include "Happen to (someone)" , "Happen, occur" , "Turn out" .)