Hardly ever (6)
'hardly ever' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rarely that I've seen before include "Not done or occurring very often" , "Very occasionally" , "Not very often, seldom in fact" , "when hundred-to-one horse wins?" , "very well" .)
(Other definitions for seldom that I've seen before include "on rare occasions" , "Models (anag)" , "Not 21" , "Not very often, rarely" , "Alluvium" .)