Very seldom (6)
I believe the answer is:
rarely
'very seldom' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rarely that I've seen before include "when hundred-to-one horse wins?" , "once in a blue moon" , "Gas used in fluorescent signs" , "Very occasionally" , "Not very often, seldom in fact" .)